Man: fix defaults for zfs_dirty_data_max_max

It was changed in e99932f7dec6efeb006e225e0bf0901c30345cac,
but without docs update.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #14400