Fix typos in zpool_do_scrub() error messages (#17028)

Sponsored-by: Wasabi Technology, Inc.
Sponsored-by: Klara, Inc.

Signed-off-by: Mateusz Piotrowski <0mp@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>