Simplify example for nn.Flatten (#67472)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/67415
Using the docstring example provided by jbschlosser to the issue submitted by qzylalala
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67472
Reviewed By: soulitzer
Differential Revision: D32210995
Pulled By: jbschlosser
fbshipit-source-id: f22bcd729699993942b6e676b479618ac613022c
