Corrected typo: maked → masked
# What does this PR do?

Fixed small typo in the BERT documentation.

## Before submitting
✅ This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

@LysandreJik @sgugger
Ugh and I copied that docstrings to all other models... Thanks for fixing!