[media] V4L2 spec: fix the description of V4L2_FBUF_CAP_SRC_CHROMAKEY

V4L2_FBUF_CAP_SRC_CHROMAKEY is the opposite of V4L2_FBUF_CAP_CHROMAKEY, but
according to the documentation it was the same. The terms 'video' and
'framebuffer' were accidentally swapped in the text.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
CC: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>