Remove preview URL reference from runtime-overview.md (#931)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/931

This was the only reference I found to the preview hostname in the tree.

Reviewed By: mergennachin

Differential Revision: D50307683

fbshipit-source-id: b20847d8b5ca6a13f881708fa8bad809b29dddb7