Fixed changing stat boost by negative actually increasing it.

src/Battling/Models/Creature.cpp

@@ -45,7 +45,7 @@ void Battling::Creature::ChangeStatBoost(Core::Statistic stat, int8_t diffAmount
     if (diffAmount > 0)
         this->_statBoost.IncreaseStatBy(stat, diffAmount);
     else
-        this->_statBoost.DecreaseStatBy(stat, diffAmount);
+        this->_statBoost.DecreaseStatBy(stat, -diffAmount);
     this->RecalculateBoostedStat(stat);
 }