media: v4l: vidioc-prepare-buf.rst: fix link to VIDIOC_QBUF

The description for VIDIOC_PREPARE_BUF results in the following
sentence: "...before actually enqueuing it, using the ioctl VIDIOC_QBUF,
VIDIOC_DQBUF ioctl...".

The intent is to only refer to VIDIOC_QBUF though, so fix this.

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