fix missed BartForMaskedLM renaming
Quick fix @sshleifer 
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3149?src=pr&el=h1) Report
> Merging [#3149](https://codecov.io/gh/huggingface/transformers/pull/3149?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/857e0a0d3ba39be6259961524a730d3f106cec9c?src=pr&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3149/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/3149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3149      +/-   ##
==========================================
- Coverage   78.03%   77.97%   -0.06%     
==========================================
  Files          98       98              
  Lines       16588    16588              
==========================================
- Hits        12944    12935       -9     
- Misses       3644     3653       +9
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `91.08% <0%> (-2.13%)` | :arrow_down: |
| [src/transformers/modeling\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3149/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ191dGlscy5weQ==) | `94.56% <0%> (+0.15%)` | :arrow_up: |

------

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<|||||>Thanks!