fix typo: "mow" -> "now"

I noticed a typo as I was reading the datasets documentation. This PR contains a trivial fix changing "mow" to "now."

_The documentation is not available anymore as the PR was closed or merged._

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.006804 / 0.011353 (-0.004549) | 0.004984 / 0.011008 (-0.006024) | 0.096781 / 0.038508 (0.058273) | 0.033049 / 0.023109 (0.009939) | 0.297681 / 0.275898 (0.021783) | 0.329553 / 0.323480 (0.006073) | 0.005697 / 0.007986 (-0.002289) | 0.004019 / 0.004328 (-0.000310) | 0.072691 / 0.004250 (0.068441) | 0.046921 / 0.037052 (0.009868) | 0.311467 / 0.258489 (0.052978) | 0.337616 / 0.293841 (0.043775) | 0.042400 / 0.128546 (-0.086146) | 0.011919 / 0.075646 (-0.063727) | 0.331390 / 0.419271 (-0.087881) | 0.051004 / 0.043533 (0.007471) | 0.295317 / 0.255139 (0.040178) | 0.316570 / 0.283200 (0.033371) | 0.099283 / 0.141683 (-0.042400) | 1.430583 / 1.452155 (-0.021572) | 1.493550 / 1.492716 (0.000834) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.213634 / 0.018006 (0.195628) | 0.432557 / 0.000490 (0.432067) | 0.001586 / 0.000200 (0.001386) | 0.000079 / 0.000054 (0.000025) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.025249 / 0.037411 (-0.012162) | 0.105433 / 0.014526 (0.090908) | 0.113474 / 0.176557 (-0.063082) | 0.168799 / 0.737135 (-0.568336) | 0.119363 / 0.296338 (-0.176975) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.412450 / 0.215209 (0.197241) | 4.117432 / 2.077655 (2.039777) | 1.935176 / 1.504120 (0.431056) | 1.745674 / 1.541195 (0.204479) | 1.853872 / 1.468490 (0.385382) | 0.703429 / 4.584777 (-3.881348) | 3.756981 / 3.745712 (0.011269) | 3.730607 / 5.269862 (-1.539255) | 1.839052 / 4.565676 (-2.726624) | 0.087574 / 0.424275 (-0.336701) | 0.012293 / 0.007607 (0.004686) | 0.517234 / 0.226044 (0.291190) | 5.189759 / 2.268929 (2.920831) | 2.418739 / 55.444624 (-53.025885) | 2.081424 / 6.876477 (-4.795053) | 2.204464 / 2.142072 (0.062392) | 0.842768 / 4.805227 (-3.962459) | 0.169014 / 6.500664 (-6.331650) | 0.063711 / 0.075469 (-0.011758) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.180636 / 1.841788 (-0.661152) | 14.816088 / 8.074308 (6.741779) | 14.290085 / 10.191392 (4.098693) | 0.165267 / 0.680424 (-0.515156) | 0.017290 / 0.534201 (-0.516911) | 0.419678 / 0.579283 (-0.159605) | 0.418164 / 0.434364 (-0.016200) | 0.492210 / 0.540337 (-0.048127) | 0.588528 / 1.386936 (-0.798408) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.007144 / 0.011353 (-0.004209) | 0.005223 / 0.011008 (-0.005785) | 0.073583 / 0.038508 (0.035075) | 0.033534 / 0.023109 (0.010425) | 0.339020 / 0.275898 (0.063122) | 0.366546 / 0.323480 (0.043066) | 0.006245 / 0.007986 (-0.001741) | 0.004081 / 0.004328 (-0.000247) | 0.073089 / 0.004250 (0.068839) | 0.047024 / 0.037052 (0.009971) | 0.342540 / 0.258489 (0.084051) | 0.379743 / 0.293841 (0.085902) | 0.037551 / 0.128546 (-0.090995) | 0.012246 / 0.075646 (-0.063400) | 0.084796 / 0.419271 (-0.334476) | 0.052256 / 0.043533 (0.008723) | 0.342675 / 0.255139 (0.087536) | 0.367157 / 0.283200 (0.083957) | 0.102939 / 0.141683 (-0.038744) | 1.409039 / 1.452155 (-0.043115) | 1.526137 / 1.492716 (0.033420) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.208143 / 0.018006 (0.190136) | 0.437940 / 0.000490 (0.437450) | 0.000424 / 0.000200 (0.000224) | 0.000056 / 0.000054 (0.000002) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.028321 / 0.037411 (-0.009091) | 0.110417 / 0.014526 (0.095891) | 0.119449 / 0.176557 (-0.057107) | 0.168081 / 0.737135 (-0.569054) | 0.126658 / 0.296338 (-0.169681) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.429302 / 0.215209 (0.214093) | 4.270547 / 2.077655 (2.192892) | 2.061323 / 1.504120 (0.557203) | 1.857877 / 1.541195 (0.316682) | 1.873317 / 1.468490 (0.404827) | 0.688750 / 4.584777 (-3.896027) | 3.767951 / 3.745712 (0.022239) | 2.011436 / 5.269862 (-3.258426) | 1.299965 / 4.565676 (-3.265712) | 0.084799 / 0.424275 (-0.339476) | 0.012082 / 0.007607 (0.004475) | 0.521981 / 0.226044 (0.295937) | 5.265333 / 2.268929 (2.996405) | 2.494326 / 55.444624 (-52.950298) | 2.144672 / 6.876477 (-4.731804) | 2.365624 / 2.142072 (0.223551) | 0.839868 / 4.805227 (-3.965359) | 0.166614 / 6.500664 (-6.334050) | 0.063804 / 0.075469 (-0.011665) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.264623 / 1.841788 (-0.577164) | 14.946515 / 8.074308 (6.872207) | 14.450115 / 10.191392 (4.258723) | 0.163878 / 0.680424 (-0.516546) | 0.017501 / 0.534201 (-0.516700) | 0.420992 / 0.579283 (-0.158291) | 0.423005 / 0.434364 (-0.011359) | 0.489505 / 0.540337 (-0.050832) | 0.594631 / 1.386936 (-0.792305) |

</details>
</details>

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