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.