Remove extra close bracket in extending.rst (#55409)
Summary:
Small typo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55409
Reviewed By: pbelevich
Differential Revision: D27611177
Pulled By: jamesr66a
fbshipit-source-id: 8a5ff702e4ab8a7eb2403432889f8b7a5a69484b
