fix cholesky call in potrs example (#15215)

fix cholesky call in potrs example (#15215)

Summary:
Cholesky by default returns the lower triangular matrix, see [docs](https://pytorch.org/docs/stable/torch.html#torch.cholesky).

However `torch.potrs` by default requires the upper triangular matrix. The naming of the variable `u` suggests that the example expects the upper to be returned, so I've added the flag to make that happen in the example.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15215

Differential Revision: D13476468

Pulled By: soumith

fbshipit-source-id: 7b68035f435a2b1be4d363b3f63e407394af949d