media: v4l2-async: Fix trivial documentation typo

Fix the incorrect spelling asyncrhronous as asynchronous, which is
visible in the public documentation of enum v4l2_async_match_type.

Fixes: ab4f5a4a

 ("[media] v4l2-async: document the remaining stuff")
Signed-off-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>