[media] DocBook: Change DTD schema to version 4.5

According with the docs at docbook.org, no backward compatible
changes were done between 4.2 and 4.5 schemas. Some fixes were
added, together with new features. So, let's use the latest
4.x schema.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media_api.tmpl

 <?xml version="1.0" encoding="UTF-8"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
-	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+	"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
 <!ENTITY % media-entities SYSTEM "./media-entities.tmpl"> %media-entities;
 <!ENTITY media-indices SYSTEM "./media-indices.tmpl">