CREATE POLICY expression -> using_expression
CREATE POLICY expression -> using_expression

The syntax for CREATE POLICY simply used "expression" for the USING
expression, while the WITH CHECK expression was "check_expression".
Given that we have two expressions, it's sensible to explcitly name both
to maintain clarity.

This patch simply changes the generic "expression" to be
"using_expression".

Pointed out by Peter Geoghegan.

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