Improve "Heaven's hammer" 20 drop table

Currently this is the drop table for this skill. It is only drop by boss monster. Compare this to "Genesis" 20 and 30 which is both drop by `Oblivion Guardian` it is really difficult to obtain. https://uploads.playkuro.com/forum/images/1778246734813-beb5a9882c80a386.png Below is 30 mastery book drop table for HH: https://uploads.playkuro.com/forum/images/1778246944930-fd4600bf974ef9c8.png Suggest to add mastery book 20, to skelosarus.