[media] pixfmt-006.rst: add missing V4L2_YCBCR_ENC_SMPTE240M

Fix missing documentation, and its cross reference.

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