Documentation: media: camera-sensor: Fix typo and vocabulary selection

Drop an unneeded double colon, and use 'shall' instead of 'must' for
consistency with the rest of the file.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>