Fixed typo in migration guide
This PR fixes a minor typo in the migration guide. `weights` was misspelled as `weigths`
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/977?src=pr&el=h1) Report
> Merging [#977](https://codecov.io/gh/huggingface/pytorch-transformers/pull/977?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/4fc9f9ef54e2ab250042c55b55a2e3c097858cb7?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/977/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/977?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master     #977   +/-   ##
=======================================
  Coverage   79.16%   79.16%           
=======================================
  Files          38       38           
  Lines        6406     6406           
=======================================
  Hits         5071     5071           
  Misses       1335     1335
```



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> Powered by [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/977?src=pr&el=footer). Last update [4fc9f9e...a6f412d](https://codecov.io/gh/huggingface/pytorch-transformers/pull/977?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Ok, thanks!