zfs.4: Correct the default for zfs_vdev_async_write_max_active

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #14179