Fixing a broken link in the README.md
Fixing the `Quick tour` link.
Thanks Gregory :)<|||||># [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=h1) Report
> Merging [#930](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/6b763d04a930e070e4096fefa1bbdb50f0575d52?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #930   +/-   ##
=======================================
  Coverage   79.04%   79.04%           
=======================================
  Files          34       34           
  Lines        6242     6242           
=======================================
  Hits         4934     4934           
  Misses       1308     1308
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?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-transformers/pull/930?src=pr&el=footer). Last update [6b763d0...4e8c1f6](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||># [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=h1) Report
> Merging [#930](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/6b763d04a930e070e4096fefa1bbdb50f0575d52?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #930   +/-   ##
=======================================
  Coverage   79.04%   79.04%           
=======================================
  Files          34       34           
  Lines        6242     6242           
=======================================
  Hits         4934     4934           
  Misses       1308     1308
```



------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?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-transformers/pull/930?src=pr&el=footer). Last update [6b763d0...4e8c1f6](https://codecov.io/gh/huggingface/pytorch-transformers/pull/930?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
