In contrast to all other colorspaces, the BT.2020 colorspace uses
limited range R'G'B' quantization as the default.

This was incorrected documented, so fix this.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>