media: vb2: fix confusing log message

If the number of allocated buffers is less than q->min_queued_buffers,
then a debug message was logged saying that it needs at least that
many queued buffers. But the test is about allocated buffers.

Update the message to say "allocated buffers".

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Acked-by: Tomasz Figa <tfiga@chromium.org>