Fix variable names in torch.fft examples (#57290)
Summary:
Apparently normal reST doctests aren't run in CI, because of this line in the `conf.py`:
https://github.com/pytorch/pytorch/blob/ac86e0a0e5ee805592a7804eee05ed8d5be85e5a/docs/source/conf.py#L366
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57290
Reviewed By: astaff
Differential Revision: D28118198
Pulled By: mruberry
fbshipit-source-id: 7af621c4fef4e5d37e0fc62b9fd4382cc1698d89
