fix typo in readme: extract_classif.py ==> extract_features.py
There seems to be a typo in the `README.md` file in Section `Example` (as shown in the following figure), I guess the script name should be `extract_features.py`.


![123456](https://user-images.githubusercontent.com/2620608/61047755-c865eb80-a412-11e9-9060-3e9f1e423d53.png)

# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=h1) Report
> Merging [#775](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/78462aad6113d50063d8251e27dbaadb7f44fbf0?src=pr&el=desc) will **decrease** coverage by `0.1%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master     #775      +/-   ##
==========================================
- Coverage    61.5%   61.39%   -0.11%     
==========================================
  Files          19       19              
  Lines        4026     4025       -1     
==========================================
- Hits         2476     2471       -5     
- Misses       1550     1554       +4
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/file\_utils.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvZmlsZV91dGlscy5weQ==) | `66.44% <0%> (-1.35%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/optimization.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvb3B0aW1pemF0aW9uLnB5) | `73.52% <0%> (-0.74%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization\_gpt2.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX2dwdDIucHk=) | `81.91% <0%> (-0.54%)` | :arrow_down: |
| [pytorch\_pretrained\_bert/tokenization\_transfo\_xl.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvdG9rZW5pemF0aW9uX3RyYW5zZm9feGwucHk=) | `31.85% <0%> (-0.19%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=footer). Last update [78462aa...b72f755](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/775?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
