docs: remove "III" version text from pgAdmin link

Reported-by: vodevsh@gmail.com

Discussion: https://postgr.es/m/152404286919.19366.7988650271505173666@wrigleys.postgresql.org

Backpatch-through: 9.3

doc/src/sgml/external-projects.sgml

@@ -146,7 +146,7 @@
   <para>
    There are several administration tools available for
    <productname>PostgreSQL</productname>. The most popular is
-   <application><ulink url="http://www.pgadmin.org/">pgAdmin III</ulink></application>,
+   <application><ulink url="http://www.pgadmin.org/">pgAdmin</ulink></application>,
    and there are several commercially available ones as well.
   </para>
  </sect1>