fix torch.frac documentation (#23830)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/13968 .
Following the math formula in wiki: https://en.wikipedia.org/wiki/Fractional_part
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23830
Differential Revision: D16656871
Pulled By: ailzhang
fbshipit-source-id: a71467870cf9566e0c7b1a045f72607dada81e1f
