[media] subdev-formats.rst: remove spurious '-'

Remove spurious duplicate '-' in the Bayer Formats description. This resulted in a
weird dot character that also caused the row to be double-height.

The - character was probably used originally as indicator of an unused bit, but as the
number of columns was increased it was never used for the new columns.

Other tables do not use '-' either, so just remove it.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>