Fix typo in extending.rst (#55408)
Summary:
Small typo in docs
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55408
Reviewed By: pbelevich
Differential Revision: D27611175
Pulled By: jamesr66a
fbshipit-source-id: a83a6220054c0411329792c7ac6afceb2b699f44
