Fix typo in code example in docs


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<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) | 0.011548 / 0.011353 (0.000196) | 0.004630 / 0.011008 (-0.006378) | 0.105349 / 0.038508 (0.066841) | 0.110557 / 0.023109 (0.087448) | 0.395463 / 0.275898 (0.119565) | 0.448391 / 0.323480 (0.124912) | 0.005112 / 0.007986 (-0.002873) | 0.003854 / 0.004328 (-0.000474) | 0.088513 / 0.004250 (0.084263) | 0.073081 / 0.037052 (0.036028) | 0.391572 / 0.258489 (0.133083) | 0.459543 / 0.293841 (0.165702) | 0.040424 / 0.128546 (-0.088122) | 0.010306 / 0.075646 (-0.065340) | 0.365493 / 0.419271 (-0.053778) | 0.068154 / 0.043533 (0.024622) | 0.397675 / 0.255139 (0.142536) | 0.447147 / 0.283200 (0.163947) | 0.033482 / 0.141683 (-0.108201) | 1.857087 / 1.452155 (0.404932) | 1.973311 / 1.492716 (0.480595) |

### 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) | 0.257938 / 0.018006 (0.239932) | 0.569572 / 0.000490 (0.569083) | 0.012155 / 0.000200 (0.011955) | 0.000112 / 0.000054 (0.000058) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.033094 / 0.037411 (-0.004318) | 0.102370 / 0.014526 (0.087844) | 0.122421 / 0.176557 (-0.054136) | 0.189983 / 0.737135 (-0.547152) | 0.117902 / 0.296338 (-0.178437) |

### 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) | 0.468419 / 0.215209 (0.253210) | 4.671410 / 2.077655 (2.593755) | 2.371136 / 1.504120 (0.867016) | 2.191877 / 1.541195 (0.650682) | 2.301894 / 1.468490 (0.833404) | 0.572260 / 4.584777 (-4.012517) | 4.302031 / 3.745712 (0.556319) | 4.128431 / 5.269862 (-1.141431) | 2.464543 / 4.565676 (-2.101133) | 0.067663 / 0.424275 (-0.356612) | 0.008947 / 0.007607 (0.001340) | 0.570063 / 0.226044 (0.344018) | 5.684460 / 2.268929 (3.415531) | 2.969708 / 55.444624 (-52.474916) | 2.573568 / 6.876477 (-4.302909) | 2.666074 / 2.142072 (0.524001) | 0.710098 / 4.805227 (-4.095129) | 0.158413 / 6.500664 (-6.342251) | 0.072776 / 0.075469 (-0.002693) |

### 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) | 1.564166 / 1.841788 (-0.277622) | 23.612774 / 8.074308 (15.538465) | 17.725070 / 10.191392 (7.533678) | 0.178982 / 0.680424 (-0.501442) | 0.021615 / 0.534201 (-0.512586) | 0.467090 / 0.579283 (-0.112193) | 0.472648 / 0.434364 (0.038284) | 0.578820 / 0.540337 (0.038483) | 0.783533 / 1.386936 (-0.603403) |

</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) | 0.008895 / 0.011353 (-0.002458) | 0.004617 / 0.011008 (-0.006392) | 0.077677 / 0.038508 (0.039169) | 0.090283 / 0.023109 (0.067174) | 0.491115 / 0.275898 (0.215217) | 0.525189 / 0.323480 (0.201709) | 0.007845 / 0.007986 (-0.000141) | 0.003742 / 0.004328 (-0.000586) | 0.077856 / 0.004250 (0.073606) | 0.067447 / 0.037052 (0.030394) | 0.488423 / 0.258489 (0.229933) | 0.532938 / 0.293841 (0.239097) | 0.041035 / 0.128546 (-0.087511) | 0.009917 / 0.075646 (-0.065730) | 0.085313 / 0.419271 (-0.333958) | 0.063374 / 0.043533 (0.019841) | 0.472287 / 0.255139 (0.217148) | 0.509773 / 0.283200 (0.226573) | 0.028706 / 0.141683 (-0.112977) | 1.775558 / 1.452155 (0.323403) | 1.967778 / 1.492716 (0.475061) |

### 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) | 0.249834 / 0.018006 (0.231828) | 0.467266 / 0.000490 (0.466776) | 0.005837 / 0.000200 (0.005637) | 0.000128 / 0.000054 (0.000074) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.038759 / 0.037411 (0.001347) | 0.113156 / 0.014526 (0.098630) | 0.123936 / 0.176557 (-0.052621) | 0.186831 / 0.737135 (-0.550304) | 0.125195 / 0.296338 (-0.171143) |

### 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) | 0.545666 / 0.215209 (0.330457) | 5.465713 / 2.077655 (3.388058) | 2.941279 / 1.504120 (1.437159) | 2.688377 / 1.541195 (1.147182) | 2.619501 / 1.468490 (1.151010) | 0.577974 / 4.584777 (-4.006803) | 4.300966 / 3.745712 (0.555254) | 3.879552 / 5.269862 (-1.390310) | 2.454932 / 4.565676 (-2.110745) | 0.069233 / 0.424275 (-0.355043) | 0.009729 / 0.007607 (0.002122) | 0.595290 / 0.226044 (0.369245) | 5.945445 / 2.268929 (3.676516) | 3.314607 / 55.444624 (-52.130017) | 2.894474 / 6.876477 (-3.982002) | 3.140790 / 2.142072 (0.998718) | 0.695808 / 4.805227 (-4.109419) | 0.158087 / 6.500664 (-6.342577) | 0.071374 / 0.075469 (-0.004095) |

### 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) | 1.706482 / 1.841788 (-0.135306) | 24.022666 / 8.074308 (15.948358) | 17.658003 / 10.191392 (7.466611) | 0.196771 / 0.680424 (-0.483653) | 0.023928 / 0.534201 (-0.510273) | 0.471992 / 0.579283 (-0.107291) | 0.510463 / 0.434364 (0.076099) | 0.621250 / 0.540337 (0.080912) | 0.807670 / 1.386936 (-0.579266) |

</details>
</details>

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