Fix syntax typo in README.md
![image](https://user-images.githubusercontent.com/27808442/65991735-8343d980-e496-11e9-90b4-bfbd61d02de0.png)

Thanks :)<|||||>Your welcome, you are doing a great job!<|||||># [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1396?src=pr&el=h1) Report
> Merging [#1396](https://codecov.io/gh/huggingface/transformers/pull/1396?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/5c3b32d44d0164aaa9b91405f48e53cf53a82b35?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1396/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1396?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1396   +/-   ##
=======================================
  Coverage   84.69%   84.69%           
=======================================
  Files          84       84           
  Lines       12596    12596           
=======================================
  Hits        10668    10668           
  Misses       1928     1928
```



------

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