[media] pixfmt-007.rst: fix copy-and-paste error in SMPTE-240M doc

Instead of referring to Y', Cb and Cr, it referred to Yc', Cbc and
Crc, which were accidentally copied from the BT.2020 constant luminance
text.

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