Fix documentation for unfold(dimension=..., ...), fixes #18793 (#19020)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19020
ghimport-source-id: 8f31e51b79daba11939aa7992450984054713b9c
Differential Revision: D14851890
Pulled By: ezyang
fbshipit-source-id: 8498e86a63633fdfd9ecae9b7f85b773b75fe27a
