media: documentation: fix video_event description

The type for the timestamp in video_event was changed to
'long' a long time ago, change the documentation to match.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>