[media] V4L: DocBook: Fix typos in the multi-plane formats description

Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/DocBook/media/v4l/pixfmt-nv12m.xml

     <refentry id="V4L2-PIX-FMT-NV12M">
       <refmeta>
-	<refentrytitle>V4L2_PIX_FMT_NV12M ('NV12M')</refentrytitle>
+	<refentrytitle>V4L2_PIX_FMT_NV12M ('NM12')</refentrytitle>
 	&manvol;
       </refmeta>
       <refnamediv>

Documentation/DocBook/media/v4l/pixfmt-yuv420m.xml

     <refentry id="V4L2-PIX-FMT-YUV420M">
       <refmeta>
-	<refentrytitle>V4L2_PIX_FMT_YUV420M ('YU12M')</refentrytitle>
+	<refentrytitle>V4L2_PIX_FMT_YUV420M ('YM12')</refentrytitle>
 	&manvol;
       </refmeta>
       <refnamediv>