Fix typo in comment in cpp_extension (#30028)
Summary:
From https://github.com/pytorch/pytorch/issues/26614
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30028
Differential Revision: D18597666
Pulled By: albanD
fbshipit-source-id: 93bf0e4ee34a63df4b544d44f630a9c0fc95fd83
