[media] DocBook media: fix xvYCC601 documentation

The documentation of the xvYCC601 Y'CbCr encoding was part of the SMPTE 170M
(SDTV) colorspace, but it should have been part of the Rec. 709 (HDTV) colorspace
as per the xvYCC standard.

This change only affects the documentation and not any code.

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