2618 arc.c mistypes in the comments

Reviewed by: Jason King <jason.brian.king@gmail.com>
Reviewed by: Josef Sipek <jeffpc@josefsipek.net>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Bart Coddens <bart.coddens@gmail.com>

illumos/illumos-gate@fc98fea58e89224f6f13d7fae246d6cb5dfa35ea