media: docs: v4l2-subdev: fix typo

Fix "will to" -> "will do".

Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>