grammatically update index.rst (#45801)
Summary:
This is a following up PR for https://github.com/pytorch/pytorch/issues/45652 which has a problem to rebase.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45801
Reviewed By: VitalyFedyunin
Differential Revision: D24111776
Pulled By: glaringlee
fbshipit-source-id: 2c727a17426be91a4df78a195de79197e1c5d120
