fix typo in GradScaler docstring (#42236)
Summary:
Closes https://github.com/pytorch/pytorch/issues/42226.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42236
Reviewed By: albanD
Differential Revision: D22817980
Pulled By: ngimel
fbshipit-source-id: 4326fe028dba1dbeed454edc4e4d4fffa56f51d6
