vty-ref: Update URI of docbook 5.0 schema

... to match the /etc/xml/catalog file on debian (no "www" in hostname)

Change-Id: Id9f3579c7f2bc3af13fe30b5268f249b6f59ed0d

doc/manuals/osmomsc-vty-reference.xml

@@ -3,8 +3,8 @@
   ex:ts=2:sw=42sts=2:et
   -*- tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*-
 -->
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V5.0//EN"
-"http://www.docbook.org/xml/5.0/dtd/docbook.dtd" [
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML 5.0//EN"
+"http://docbook.org/xml/5.0/dtd/docbook.dtd" [
 <!ENTITY chapter-vty      SYSTEM      "../common/chapters/vty.xml" >
 <!ENTITY sections-vty     SYSTEM      "generated/docbook_vty.xml"  >
 ]>