Use gunmod dispersion, display aim-speed as indicated.

In cef874e2f1f08f50a32bbc56838831f4818a5242, `it_gunmod::mod_dispersion` was added, loaded from the json member "dispersion". The new member was not actually used in that commit, but a usage of `it_gunmod::dispersion` was added (in `item::dispersion`). Was `mod_dispersion` meant? Because `mod_dispersion` is not used anywhere (it's loaded from json, and displayed in the item info, that's all).

Also fixes the display of "Aim speed" (was showing the wrong member).