Fix Bugged Vendor Level Availability of Amulets and Belts

As discovered in Race Discussion, and laid out in this post, some Amulet and Belt vendor availability levels were screwed up at some point.

Agate/Citrine/Turquoise Amulet: available at character level 19, should be available at 15.

Leather Belt, Heavy Belt: available at character level 9, should be available at 7.