Fix docstring for nn.Softplus (#70576)
Summary:
Fixes nn.Softplus' docstring problem reported at https://github.com/pytorch/pytorch/issues/70498.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70576
Reviewed By: VitalyFedyunin
Differential Revision: D33407444
Pulled By: albanD
fbshipit-source-id: 7f1f438afb1a1079d30e0c4741aa609c5204329f
