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>