media: docs: v4l2-controls: fix sentence rendered in a nonsense way

This sentence renders as:

> Since such compound controls need to expose more information about
> themselves than is possible with ioctls VIDIOC_QUERYCTRL,
> VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU the VIDIOC_QUERY_EXT_CTRL
  ^^^^^^^^^^^^^^^^^^^^^                          ^^^^^^^^^^^^^^^^^^^^^
> ioctl was added.

This does not make sense. Fix by providing an explicit link text. This
results in:

> Since such compound controls need to expose more information about
> themselves than is possible with VIDIOC_QUERYCTRL the
> VIDIOC_QUERY_EXT_CTRL ioctl was added.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>