If it's (on), it's off the list

Because NPCs don't set their (on) items to actively use charges.  If you
were to obtain such an item, it'd be permanently active and consume no
charges.

So they're on the trader_avoid list.
Gappy armor is the worst!

Fixes typo.
Merge pull request #6715 from KA101/TraderAvoidOn

If it's (on), it's off the list