[media] DocBook media: fix BT.2020 description

One number was wrong (0.6789 -> 0.6780) and Y' should have been Yc'.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
1 file changed