[torch.futures] Fix doc inconsistency about callback args (#50979)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50979
Noticed that the documentation is inconsisntent about the arg needed
in the callback. It appears to require the future, so fix this in the docs.
ghstack-source-id: 121021431
Test Plan: CI
Reviewed By: mrshenli
Differential Revision: D25944637
fbshipit-source-id: 0bfcd4040c4a1c245314186d29a0031e634b29c3