Fix docs for ShardMetadata. (#61388)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61388
The doc for `placement` argument was outdated and is now fixed.
ghstack-source-id: 133184441
Test Plan: waitforbuildbot
Reviewed By: wanchaol
Differential Revision: D29601316
fbshipit-source-id: a0817f799382bf91a5192c54dfeea4d253eb0d56
