fix typo in adam docs (#70387)
Summary:
Fix the typo in [adam docs in master branch](https://pytorch.org/docs/master/generated/torch.optim.Adam.html#torch.optim.Adam)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70387
Reviewed By: H-Huang
Differential Revision: D33309283
Pulled By: albanD
fbshipit-source-id: d20c5d8f2498ac64013f71e202a6b50dcc069f2b
