Modify torch.gesv error message (#15654)
Summary:
[doc](https://pytorch.org/docs/stable/torch.html#torch.gesv) uses `B` uppercase so error message should follow to avoid confusion.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15654
Differential Revision: D13571297
Pulled By: soumith
fbshipit-source-id: 0b4e7797eceff92618f808bbfa65d13c1dcc2da0
