Set `discountPerBlock` to 0.003 ETH

Right now the discountPerBlock is 0.002 ETH. Meaning the maximum discount for a corgi is ~0.5 ETH (with a 0.01 floor price). By setting this parameter to 0.003 ETH we make that max discount 0.75 ETH.

In practice, with the 0.002 ETH discount, corgis will start climbing in price again once ~5000 are sold (starting from 0.01 ETH). With the 0.003 ETH discount, the floor price will remain for old corgis until ~7500 are sold.