Incorrect docstring for BertForMaskedLM
The docstring for the head_mask argument to the BertForMaskedLM class is repeated and one is incorrect - I presume it's just a copy-paste mistake.
# [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=h1) Report
> Merging [#718](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/commit/98dc30b21e3df6528d0dd17f0910ffea12bc0f33?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #718   +/-   ##
=======================================
  Coverage   62.22%   62.22%           
=======================================
  Files          18       18           
  Lines        3979     3979           
=======================================
  Hits         2476     2476           
  Misses       1503     1503
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_pretrained\_bert/modeling.py](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718/diff?src=pr&el=tree#diff-cHl0b3JjaF9wcmV0cmFpbmVkX2JlcnQvbW9kZWxpbmcucHk=) | `79.49% <ø> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=footer). Last update [98dc30b...8d6a118](https://codecov.io/gh/huggingface/pytorch-pretrained-BERT/pull/718?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks, we'll make a big clean up of the docstrings for the coming release.