Remove ambiguity on demand vs prefetch stats reported by arc_summary

arc_summary currently list prefetch stats as "demand prefetch"
However, a hit/miss can be due to demand or prefetch, not both.
To remove any confusion, this patch removes the "Demand" word
from the affected lines.

Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Gionatan Danti <g.danti@assyoma.it>
Closes #13985