greybus: Documentation/sysfs-bus-greybus: update kernel version and date

The kernel is now on the 4.XX numbering scheme, and it's going to be a
while before we merge this code, so pick a date sometime in the future
to be safe.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>