Update README with new update command (#9203)

Note: `upgrade_oh_my_zsh` is deprecated. Use `omz update` instead.

README.md

@@ -273,7 +273,7 @@ DISABLE_AUTO_UPDATE=true
 If you'd like to upgrade at any point in time (maybe someone just released a new plugin and you don't want to wait a week?) you just need to run:
 
 ```shell
-upgrade_oh_my_zsh
+omz update
 ```
 
 Magic! 🎉