adjust lich primary spell weights

i had doubled up greater demon earlier because 20% seemed a bit low, but
25% is probably fine, especially since we are still getting comments
about randliches being pretty summon-heavy. note that because of the
chance of ancient liches getting two primary spells, the chance for
greater demon on ancient liches actually goes from 47% to 38% with this
change, compared to 33% -> 25% on normal liches.