`torch.matrix_exp` doc fix (#45909)
Summary:
As per title.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45909
Reviewed By: dzhulgakov
Differential Revision: D24147314
Pulled By: albanD
fbshipit-source-id: fc21094f4dbdd04cc2063a9639b9d1f5728cb53f
