[FSDP][Easy] Remove outdated comment (#92739)
We pass `fully_sharded_module`, not `root_module`, after recent refactoring to unify composable and wrapper FSDP for now. This PR removes the comment explaining why before we passed in `root_module`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92739
Approved by: https://github.com/mrshenli
