[media] V4L: fix the Z16 format definition

A copy paste error created that format with the same one-line
description as Y8I and Y12I, whereas Z16 is quite different from them
both.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/DocBook/media/v4l/pixfmt-z16.xml

@@ -5,7 +5,7 @@
   </refmeta>
   <refnamediv>
     <refname><constant>V4L2_PIX_FMT_Z16</constant></refname>
-    <refpurpose>Interleaved grey-scale image, e.g. from a stereo-pair</refpurpose>
+    <refpurpose>16-bit depth data with distance values at each pixel</refpurpose>
   </refnamediv>
   <refsect1>
     <title>Description</title>