media: Documentation: v4l: fix section depth

The request API documentation introduced a new section which should have
been a subsection. Fix this.

Signed-off-by: Alexandre Courbot <acourbot@chromium.org>
Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>