Fix gram.y comment to match reality
Fix gram.y comment to match reality

There are other comments in there that don't precisely match what's
implemented, but this one confused me enough to be worth fixing.

50wu/gpdb,zeroae/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,50wu/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,yazun/postgres-xl,yazun/postgres-xl,Postgres-XL/Postgres-XL,greenplum-db/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,zeroae/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,yazun/postgres-xl,oberstet/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,ovr/postgres-xl,techdragon/Postgres-XL,pavanvd/postgres-xl,Postgres-XL/Postgres-XL,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,zeroae/postgres-xl,Postgres-XL/Postgres-XL,adam8157/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,adam8157/gpdb,ovr/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,greenplum-db/gpdb,yazun/postgres-xl,techdragon/Postgres-XL,zeroae/postgres-xl,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,oberstet/postgres-xl,adam8157/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,50wu/gpdb,50wu/gpdb,techdragon/Postgres-XL,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,oberstet/postgres-xl,ashwinstar/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,yazun/postgres-xl,xinzweb/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,oberstet/postgres-xl,greenplum-db/gpdb