[media] DocBook media: fix incorrect note about packed RGB and colorspace

The fact that the pixelformat is using a packed RGB format has nothing
to do with the colorspace that is being used. Those are very different
things. The colorspace decides what color a triplet of RGB numbers
actually map to. E.g. a red color with values (255, 0, 0) is a different
type of red depending on the colorspace. If the original pixelformat was
e.g. YUV in colorspace REC709, then after the conversion to RGB the
colorspace is still REC709. Unless the hardware actually converted the
colorspace as well from REC709 to sRGB, but that rarely if ever happens.

Remove this incorrect comment.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>