Fix typo in `Dataset.map` docstring



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<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) | 0.006709 / 0.011353 (-0.004643) | 0.004102 / 0.011008 (-0.006906) | 0.084449 / 0.038508 (0.045941) | 0.076078 / 0.023109 (0.052969) | 0.319831 / 0.275898 (0.043933) | 0.359918 / 0.323480 (0.036438) | 0.006092 / 0.007986 (-0.001894) | 0.003402 / 0.004328 (-0.000926) | 0.064715 / 0.004250 (0.060465) | 0.054541 / 0.037052 (0.017488) | 0.330394 / 0.258489 (0.071905) | 0.366048 / 0.293841 (0.072207) | 0.031594 / 0.128546 (-0.096952) | 0.008591 / 0.075646 (-0.067056) | 0.292983 / 0.419271 (-0.126288) | 0.052986 / 0.043533 (0.009453) | 0.322253 / 0.255139 (0.067114) | 0.340082 / 0.283200 (0.056882) | 0.023390 / 0.141683 (-0.118293) | 1.459038 / 1.452155 (0.006883) | 1.536256 / 1.492716 (0.043540) |

### 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) | 0.233527 / 0.018006 (0.215521) | 0.459145 / 0.000490 (0.458655) | 0.007471 / 0.000200 (0.007271) | 0.000281 / 0.000054 (0.000227) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.028158 / 0.037411 (-0.009253) | 0.083079 / 0.014526 (0.068553) | 0.097159 / 0.176557 (-0.079397) | 0.151927 / 0.737135 (-0.585208) | 0.098024 / 0.296338 (-0.198314) |

### 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) | 0.386882 / 0.215209 (0.171673) | 3.849635 / 2.077655 (1.771981) | 1.832885 / 1.504120 (0.328765) | 1.668356 / 1.541195 (0.127162) | 1.745066 / 1.468490 (0.276576) | 0.484476 / 4.584777 (-4.100301) | 3.547604 / 3.745712 (-0.198108) | 3.480338 / 5.269862 (-1.789523) | 2.066837 / 4.565676 (-2.498840) | 0.056755 / 0.424275 (-0.367520) | 0.007747 / 0.007607 (0.000140) | 0.467999 / 0.226044 (0.241955) | 4.678875 / 2.268929 (2.409946) | 2.341930 / 55.444624 (-53.102695) | 1.985632 / 6.876477 (-4.890844) | 2.046998 / 2.142072 (-0.095074) | 0.579860 / 4.805227 (-4.225367) | 0.131488 / 6.500664 (-6.369176) | 0.060193 / 0.075469 (-0.015276) |

### 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) | 1.249656 / 1.841788 (-0.592132) | 19.079517 / 8.074308 (11.005209) | 14.328827 / 10.191392 (4.137435) | 0.173707 / 0.680424 (-0.506717) | 0.018250 / 0.534201 (-0.515951) | 0.392225 / 0.579283 (-0.187058) | 0.413920 / 0.434364 (-0.020444) | 0.464124 / 0.540337 (-0.076214) | 0.640283 / 1.386936 (-0.746653) |

</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) | 0.006859 / 0.011353 (-0.004494) | 0.004068 / 0.011008 (-0.006940) | 0.063936 / 0.038508 (0.025428) | 0.077187 / 0.023109 (0.054078) | 0.365098 / 0.275898 (0.089200) | 0.391003 / 0.323480 (0.067523) | 0.005571 / 0.007986 (-0.002415) | 0.003425 / 0.004328 (-0.000904) | 0.063220 / 0.004250 (0.058970) | 0.056964 / 0.037052 (0.019912) | 0.367793 / 0.258489 (0.109304) | 0.398776 / 0.293841 (0.104935) | 0.033182 / 0.128546 (-0.095364) | 0.008601 / 0.075646 (-0.067045) | 0.070276 / 0.419271 (-0.348996) | 0.048383 / 0.043533 (0.004850) | 0.360414 / 0.255139 (0.105275) | 0.368171 / 0.283200 (0.084971) | 0.023114 / 0.141683 (-0.118569) | 1.503503 / 1.452155 (0.051349) | 1.567279 / 1.492716 (0.074562) |

### 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) | 0.224296 / 0.018006 (0.206290) | 0.455138 / 0.000490 (0.454648) | 0.004014 / 0.000200 (0.003814) | 0.000104 / 0.000054 (0.000050) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.032337 / 0.037411 (-0.005074) | 0.094385 / 0.014526 (0.079859) | 0.109870 / 0.176557 (-0.066687) | 0.156978 / 0.737135 (-0.580157) | 0.107559 / 0.296338 (-0.188780) |

### 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) | 0.427409 / 0.215209 (0.212200) | 4.261772 / 2.077655 (2.184117) | 2.276106 / 1.504120 (0.771986) | 2.115232 / 1.541195 (0.574038) | 2.192048 / 1.468490 (0.723558) | 0.488459 / 4.584777 (-4.096318) | 3.675463 / 3.745712 (-0.070249) | 3.322475 / 5.269862 (-1.947387) | 2.072253 / 4.565676 (-2.493424) | 0.058259 / 0.424275 (-0.366017) | 0.007319 / 0.007607 (-0.000288) | 0.499513 / 0.226044 (0.273469) | 4.994774 / 2.268929 (2.725845) | 2.760927 / 55.444624 (-52.683697) | 2.391947 / 6.876477 (-4.484530) | 2.600557 / 2.142072 (0.458484) | 0.587597 / 4.805227 (-4.217630) | 0.131444 / 6.500664 (-6.369220) | 0.057334 / 0.075469 (-0.018135) |

### 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) | 1.354636 / 1.841788 (-0.487152) | 19.685735 / 8.074308 (11.611427) | 14.295920 / 10.191392 (4.104528) | 0.171921 / 0.680424 (-0.508503) | 0.019926 / 0.534201 (-0.514274) | 0.395216 / 0.579283 (-0.184068) | 0.432791 / 0.434364 (-0.001573) | 0.473055 / 0.540337 (-0.067282) | 0.638633 / 1.386936 (-0.748303) |

</details>
</details>

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