Don't qualify type pg_catalog.text in extend-extensions-example.

Don't qualify type pg_catalog.text in extend-extensions-example.

Extension scripts begin execution with pg_catalog at the front of the
search path, so type names reliably refer to pg_catalog.  Remove these
superfluous qualifications.  Earlier <programlisting> of this <sect1>
already omitted them.  Back-patch to 9.3 (all supported versions).