V4L/DVB (12920): DocBook/media: Some xmlto or DTD's don't accept reference inside appendix

That was weird: on some distros, we need to remove <reference> tag
for it to work.
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/v4l/v4l2.xml

@@ -397,9 +397,8 @@ and discussions on the V4L mailing list.</revremark>
 	  &sub-compat;
   </chapter>
 
-  <appendix id="function_ref">
+  <appendix id="user-func">
   <title>Function Reference</title>
-  <reference id="user-func">
 
     <!-- Keep this alphabetically sorted. -->
 
@@ -454,7 +453,6 @@ and discussions on the V4L mailing list.</revremark>
     &sub-read;
     &sub-select;
     &sub-write;
-  </reference>
   </appendix>
 
   <appendix id="videodev">