zfs.4: Fix typo s/compatiblity/compatibility/

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Gordon Bergling <gbergling@googlemail.com>
Closes #12464