[media] Fix small DocBook typo

Here is a small patch which fixes a DocBook mistake in the decoder_cmd
documentation.
Signed-off-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/vidioc-decoder-cmd.xml

@@ -118,7 +118,7 @@ this command, drivers and applications must set this field to zero.</entry>
 	  <row>
             <entry></entry>
             <entry></entry>
-	    <entry>__u32</entry>
+	    <entry>__s32</entry>
 	    <entry><structfield>speed</structfield></entry>
             <entry>Playback speed and direction. The playback speed is defined as
 <structfield>speed</structfield>/1000 of the normal speed. So 1000 is normal playback.