Docs: fix typo in to_char() example.
Docs: fix typo in to_char() example.

Per bug #13631 from KOIZUMI Satoru.

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