Fix MultiheadAttention docstring latex (#50430)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/50429
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50430
Reviewed By: izdeby
Differential Revision: D25885695
Pulled By: zou3519
fbshipit-source-id: 7b017f9c5cdebbc7254c8193305c54003478c343
