[doc] fix typo in formula of MarginRankingLoss (#40285)
Summary:
This is just a minor doc fix:
the `MarginRankingLoss` takes 2 input samples `x1` and `x2`, not just `x`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40285
Reviewed By: ezyang
Differential Revision: D22195069
Pulled By: zou3519
fbshipit-source-id: 909f491c94dca329a37216524f4088e9096e0bc6
