fix RG_BACKSTAP (Back stab) bow damage formula

before this commit, only part of the skillratio value was being halved
when using bows, because skillratio by default is 100.

the correct behavior for bows in backstab is to halve the entire
skillratio value (including the initial 100%)

src/map/battle.c

@@ -2179,7 +2179,7 @@ static int battle_calc_skillratio(int attack_type, struct block_list *src, struc
 					break;
 				case RG_BACKSTAP:
 					if (sd != NULL && sd->weapontype == W_BOW && battle_config.backstab_bow_penalty)
-						skillratio += (200 + 40 * skill_lv) / 2;
+						skillratio += -100 + (300 + 40 * skill_lv) / 2;
 					else
 						skillratio += 200 + 40 * skill_lv;
 					break;