Instruct people to clone viable/strict as opposed to main (#6243)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6243

In the main branch of the documentation, let's ask people to use 'viable/strict' instead.

Reviewed By: kirklandsign

Differential Revision: D64375880

fbshipit-source-id: 9cb06b08ba703aaf1194be02af47214ba653419c