Fix some typos in README
This PR fixes some typos in README.md and overall makes it slightly more readable.

No code changes.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1433?src=pr&el=h1) Report
> Merging [#1433](https://codecov.io/gh/huggingface/transformers/pull/1433?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f3e0218fbb6bcc40b40f10089dae8876654edb23?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/1433/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1433?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1433   +/-   ##
=======================================
  Coverage   84.72%   84.72%           
=======================================
  Files          84       84           
  Lines       12591    12591           
=======================================
  Hits        10668    10668           
  Misses       1923     1923
```



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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1433?src=pr&el=footer). Last update [f3e0218...85d7c84](https://codecov.io/gh/huggingface/transformers/pull/1433?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks for the update!