Fix wrong link for the notebook file
For the tutorial of "How to generate text", the URL link was wrong (it was linked to the tutorial of "How to train a language model").

I fixed the URL.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3344?src=pr&el=h1) Report
> Merging [#3344](https://codecov.io/gh/huggingface/transformers/pull/3344?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/f6d813aaaa96cc43fcf55f255b9439ebc22a31a0&el=desc) will **not change** coverage by `%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3344/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3344?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #3344   +/-   ##
=======================================
  Coverage   77.63%   77.63%           
=======================================
  Files         100      100           
  Lines       16943    16943           
=======================================
  Hits        13154    13154           
  Misses       3789     3789           
```



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<|||||>Thanks a lot for pointing this out @rudvlf0413 and @julien-c 