Docs: fix misleading example.
Docs: fix misleading example.

Commit 8457d0beca731bf0 introduced an example which, while not incorrect,
failed to exhibit the behavior it meant to describe, as a result of omitting
an E'' prefix that needed to be there.  Noticed and fixed by Peter Geoghegan.

I (tgl) failed to resist the temptation to wordsmith nearby text a bit
while at it.

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