ACP links to docs need to be updated to 3.1 URLs

Here is an example of a link in the ACP which goes to the 3.0 documentation instead of the 3.1 documentation:
https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/language/en/acp/permissions.php#L57

There may be more too, that's only the one I saw though.