doc:  fix typo in CREATE POLICY manual page
doc:  fix typo in CREATE POLICY manual page

Backpatch through 9.5

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