Event core.notification_manager_add_notifications_for_users_modify_data incorrectly placed

I made a mistake in placing of this "core.notification_manager_add_notifications_for_users_modify_data" core event
Since that event is no use there for anybody , I have moved it to a more useful place.

Also Priority is major because the previous PR https://github.com/phpbb/phpbb/pull/5862  & ticket https://tracker.phpbb.com/browse/PHPBB3-16370 which I had made is in phpBB version 3.2.10-RC1 .

My apologies for the inconvenience. 

Thanking you , Best regards