[media] DocBook media: BT.2020 RGB uses limited quantization range

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>