Fix typo in comment (#20947)

Summary:
Overwrite instead of owerwrite

Release Notes:
--------------
[CLI] [MINOR] [local-cli/upgrade/upgrade.js] - Typo in comment fixed
Pull Request resolved: https://github.com/facebook/react-native/pull/20947

Differential Revision: D9661739

Pulled By: hramos

fbshipit-source-id: 378b44599313ab535af55981ddd80b0fcdbee448