Fix typo #9012 (#1)
There is a tiny typo in the code "transformers/examples/language-modeling/run_mlm_wwm.py" at line 284. [Details.](https://github.com/huggingface/transformers/issues/9012)

# What does this PR do?

Fixes #9012


## Before submitting
- [Y] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [Y] Did you write any new necessary tests?

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors which may be interested in your PR.

 albert, bert, XLM: @LysandreJik
 examples/distillation: @VictorSanh
 examples/token-classification: @stefan-it
 documentation: @sgugger
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