update hub doc (#21568)
Summary:
update doc as pointed out in https://github.com/pytorch/hub/pull/22
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21568
Differential Revision: D15732927
Pulled By: ailzhang
fbshipit-source-id: 78ab026539e5ee59e7c3a8144e2c9fcbbc225733
