MAINTAINERS: Direct Documentation/DocBook/media properly

The media maintainers want DocBook changes to go through their tree;
document that wish accordingly.

MAINTAINERS

@@ -3440,6 +3440,7 @@ X:	Documentation/devicetree/
 X:	Documentation/acpi
 X:	Documentation/power
 X:	Documentation/spi
+X:	Documentation/DocBook/media
 T:	git git://git.lwn.net/linux-2.6.git docs-next
 
 DOUBLETALK DRIVER