Fixing incorrect link in model docstring
The docstring contains a link to Salesforce/CTRL repo, while the model itself is Facebookresearch/mmbt. It may be the wrong copy\paste.
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=h1) Report
> Merging [#2260](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/645713e2cb8307e41febb2b7c9f6036f6645efce?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/2260/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #2260   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files         133      133           
  Lines       19878    19878           
=======================================
  Hits        15576    15576           
  Misses       4302     4302
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/modeling\_mmbt.py](https://codecov.io/gh/huggingface/transformers/pull/2260/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL21vZGVsaW5nX21tYnQucHk=) | `18.25% <ø> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=footer). Last update [645713e...b668a74](https://codecov.io/gh/huggingface/transformers/pull/2260?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Thanks