[PATCH] Correct RangeTblEntry.join_using_alias comment to reflect field order

Hi hackers! 

Found a tiny incorrect comment of RangeTblEntry, field "alias" is actually above "join_using_alias", so the code comment could be updated to reflect the current field order:

> join_using_alias is an alias clause attached directly to JOIN/USING. It is different from the alias field (below) in that it does not hide the

Regards,

steve