Fix incorrect CREATE STATISTICS example in docs
Fix incorrect CREATE STATISTICS example in docs

The example was incorrectly using parantheses around the list of columns, so
just drop them.

Reported-By: Robert Haas
Discussion: https://postgr.es/m/CA%2BTgmoZZEMAqWMAfvLHZnK57SoxOutgvE-ALO94WsRA7zZ7wyQ%40mail.gmail.com

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