pgsql: Replace broken link to custom local gettext package with one to

Log Message:
-----------
Replace broken link to custom local gettext package with one to the main
GNU site for gettext.

Tags:
----
REL8_4_STABLE

Modified Files:
--------------
pgsql/doc/src/sgml:
installation.sgml (r1.325.2.1 -> r1.325.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/installation.sgml?r1=1.325.2.1&r2=1.325.2.2)