[media] Fix 'ID nv12mt already defined' error

DocBook validation fix.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/v4l/pixfmt-nv12mt.xml

@@ -49,7 +49,7 @@ interleaved. Height of the buffer is aligned to 32.
 	  <title>Memory layout of macroblocks in <constant>V4L2_PIX_FMT_NV12
 </constant> format pixel image - extreme case</title>
 	<para>
-	<figure id="nv12mt">
+	<figure id="nv12mt_ex">
 	    <title>Example <constant>V4L2_PIX_FMT_NV12MT</constant> memory
 layout of macroblocks</title>
 	    <mediaobject>