fix typo in training_args.py
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/5893?src=pr&el=h1) Report
> Merging [#5893](https://codecov.io/gh/huggingface/transformers/pull/5893?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/09a2f40684f77e62d0fd8485fe9d2d610390453f&el=desc) will **increase** coverage by `0.17%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/5893/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/5893?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #5893      +/-   ##
==========================================
+ Coverage   78.49%   78.66%   +0.17%     
==========================================
  Files         146      146              
  Lines       26210    26210              
==========================================
+ Hits        20573    20619      +46     
+ Misses       5637     5591      -46     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/5893?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/training\_args.py](https://codecov.io/gh/huggingface/transformers/pull/5893/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy90cmFpbmluZ19hcmdzLnB5) | `77.55% <ø> (ø)` | |
| [src/transformers/modeling\_tf\_distilbert.py](https://codecov.io/gh/huggingface/transformers/pull/5893/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9kaXN0aWxiZXJ0LnB5) | `64.90% <0.00%> (-33.90%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_electra.py](https://codecov.io/gh/huggingface/transformers/pull/5893/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl9lbGVjdHJhLnB5) | `95.53% <0.00%> (+69.51%)` | :arrow_up: |

------

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