fix small typo in optim adamw (#40283)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/40283
Test Plan: Imported from OSS
Differential Revision: D22138796
Pulled By: glaringlee
fbshipit-source-id: 2c3a35f7e539b43ee5abf8dbc10b95df5d62fccb
