Core/Spell: fix one logic fail when calculating level penalty

(http://wow.gamepedia.com/index.php?title=Spell_power&oldid=1751770#Spells_learned_before_level_20)

src/server/game/Entities/Unit/Unit.cpp

@@ -2154,7 +2154,7 @@ float Unit::CalculateLevelPenalty(SpellInfo const* spellProto) const
     float LvlPenalty = 0.0f;
 
     if (spellProto->SpellLevel < 20)
-        LvlPenalty = 20.0f - spellProto->SpellLevel * 3.75f;
+        LvlPenalty = (20.0f - spellProto->SpellLevel) * 3.75f;
     float LvlFactor = (float(spellProto->SpellLevel) + 6.0f) / float(getLevel());
     if (LvlFactor > 1.0f)
         LvlFactor = 1.0f;