Nerf powerup frequency upgrades

Before, each one multiplied the time between spawns by 0.5
Now, they multiply the time by 0.75, then 0.78, then 0.81,
and so forth

constants.js

@@ -513,7 +513,7 @@ function createPowerupFreqUpgrades() {
         {
           "type": "powerup-freq",
           "id": prefix + counter,
-          "amount": 0.5
+          "amount": 0.75 + 0.03 * (counter - 1)
         }
       ],
       "prereqs": {