Fix adamw formula doc (#68587)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68482
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68587
Reviewed By: dagitses, jbschlosser
Differential Revision: D33478646
Pulled By: albanD
fbshipit-source-id: 4e6419829c3faa7449c041e7d467a6dab30fe917
