Fixing error in Readme.md. (#50033)
Summary:
Fix incorrect command in readme.
Fix incorrect url in readme.
Add url for dockerfile.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50033
Reviewed By: ezyang
Differential Revision: D25759567
Pulled By: mrshenli
fbshipit-source-id: 2a3bc88c8717a3890090ddd0d6657f49d14ff05a
