Add centered FFT example to fftshift docs (#51223)
Summary:
Closes https://github.com/pytorch/pytorch/issues/51022
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51223
Reviewed By: malfet
Differential Revision: D26110201
Pulled By: mruberry
fbshipit-source-id: c659c5dca30eda4b67ed6d931a93de9a33e72895
