media: buffer.rst: fix link text of VIDIOC_QBUF

The link was showing both VIDIOC_QBUF, VIDIOC_DQBUF while it should show
only VIDIOC_QBUF in this case.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>