Update the link for iOS demo app in README.md (#27145)
Summary:
Update the link for iOS demo app in README.md
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27145
Differential Revision: D17746591
Pulled By: xta0
fbshipit-source-id: 6f49a0daddc8b79804e1b8487ba1db3807a3f481