[media] DocBook: upgrade media_api DocBook version to 4.2

Fixes the last three errors of media_api DocBook validatation:
(...)
media_api.xml:414: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set
media_api.xml:432: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set
media_api.xml:452: element imagedata: validity error : Value "SVG" for attribute format of imagedata is not among the enumerated set
(...)
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Cc: stable@vger.kernel.org

Documentation/DocBook/media_api.tmpl

 <?xml version="1.0"?>
-<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
-	"http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
+<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+	"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
 <!ENTITY % media-entities SYSTEM "./media-entities.tmpl"> %media-entities;
 <!ENTITY media-indices SYSTEM "./media-indices.tmpl">