Corrected Joint Beat damage formula (#4325)

* Fixes #4305.
* Follow up to cb2c553.
* Corrected Joint Beat damage formula.
Thanks to @Indigo000!

src/map/battle.cpp

@@ -3634,7 +3634,7 @@ static int battle_calc_attack_skill_ratio(struct Damage* wd, struct block_list *
 			skillratio += 40 * skill_lv;
 			break;
 		case LK_JOINTBEAT:
-			skillratio += -100 + 10 * skill_lv - 50;
+			skillratio += 10 * skill_lv - 50;
 			if (wd->miscflag & BREAK_NECK || (tsc && tsc->data[SC_JOINTBEAT] && tsc->data[SC_JOINTBEAT]->val2 & BREAK_NECK)) // The 2x damage is only for the BREAK_NECK ailment.
 				skillratio <<= 1;
 			break;