media: videodev2.h.rst.exceptions: add V4L2_DV_FL_CAN_DETECT_REDUCED_FPS

This fixes a documentation warning:

Documentation/output/videodev2.h.rst:6: WARNING: undefined label: v4l2-dv-fl-can-detect-reduced-fps (if the link has no caption the label must precede a section header)

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>