media: ext-ctrls-codec.rst: clarify V4L2_CID_MPEG_VIDEO_BITRATE

The documentation for V4L2_CID_MPEG_VIDEO_BITRATE was missing the word
'Average': this is the average video bitrate.

The V4L2_CID_MPEG_VIDEO_BITRATE_PEAK documentation *does* refer to
V4L2_CID_MPEG_VIDEO_BITRATE as the average video bitrate.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst

@@ -580,7 +580,7 @@ enum v4l2_mpeg_video_bitrate_mode -
 
 
 ``V4L2_CID_MPEG_VIDEO_BITRATE (integer)``
-    Video bitrate in bits per second.
+    Average video bitrate in bits per second.
 
 ``V4L2_CID_MPEG_VIDEO_BITRATE_PEAK (integer)``
     Peak video bitrate in bits per second. Must be larger or equal to