Add wording consistency for update/upgrade controversy

Change main command for updating to “update_oh_my_zsh”. Seems to be
more conventional and human-readable. Some internal stuff still keeps
“upgrade”. Needs thorough testing. And maybe a transitional message for
existing users

tools/check_for_upgrade.sh

@@ -11,7 +11,7 @@ function _update_zsh_update() {
 }
 
 function _upgrade_zsh() {
-  env ZSH=$ZSH sh $ZSH/tools/upgrade.sh
+  env ZSH=$ZSH sh $ZSH/tools/update.sh
   # update the zsh file
   _update_zsh_update
 }