update exp. family doc (#18118)
Summary:
sphinx doesn't understand hyphen. it does not merge the two halves together in html.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18118
Differential Revision: D14498012
Pulled By: mrshenli
fbshipit-source-id: d6f4cfddc0a8e3a8f91578da43c26ca9c6fff3ce
