Fix function-header comments in planner.c
Fix function-header comments in planner.c

In b5635948ab1, a couple of function header comments weren't changed, or
weren't changed correctly, to reflect the arguments being passed into
the functions.  Specifically, get_number_of_groups() had the wrong
argument name in the commit and create_grouping_paths() wasn't
updated even though the arguments had been changed.

The issue with create_grouping_paths() was noticed by Ashutosh Bapat,
while I discovered the issue with get_number_of_groups() by looking to
see if there were any similar issues from that commit.

Discussion: 233046d611df63dc98adda9bc34eabffccc1b331@mail.gmail.com

adam8157/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb