[media] doc-rst: Fix number of zeroed high order bits in 12-bit raw format defs

The number of high order bits in samples was documented to be 6 for 12-bit
data. This is clearly wrong, fix it.

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