Fix incorrect variable in autograd docs (#70884)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68362.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70884
Reviewed By: mruberry
Differential Revision: D33463331
Pulled By: ngimel
fbshipit-source-id: 834ba9c450972710e0424cc92af222551f0b4a4a
