Replace `constraints` with `dynamic_shapes` in export-to-executorch tutorial (#1659)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1659

X-link: https://github.com/pytorch/pytorch/pull/117916

`constraints` argument for `torch.export` has been deprecated in favor of the `dynamic_shapes` argument. This PR updates the use of the deprecated API in export-to-executorch tutorial.

Reviewed By: angelayi

Differential Revision: D52932772

fbshipit-source-id: 5f2e2ce02ba1990a90ae3de1c6c95e767e39d298