fixed LogSigmoid math string that wasn't rendering in documentation (#16900)
Summary:
The documentation for LogSigmoid says:
> Applies the element-wise function:
> \<blank\>
Now the documentation properly displays the math string.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16900
Differential Revision: D14020097
Pulled By: ezyang
fbshipit-source-id: 41e229d0fcc6b9bb53367be548bf85286dc13546
