fixes link in TripletMarginLoss (#19417)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/19245
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19417
Differential Revision: D15001610
Pulled By: soumith
fbshipit-source-id: 1b85ebe196eb5a3af5eb83d914dafa83b9b35b31
