Add cooldown to Amplify Damage in phase 3 (#894)

Added a cooldown to amplify damage in phase 3, else it's cast every frame. Used random between 20 and 30 seconds as that's what's set at the start of phase 3.

src/server/scripts/EasternKingdoms/Karazhan/boss_prince_malchezaar.cpp

@@ -421,6 +421,7 @@ public:
 
                     if (target)
                         DoCast(target, SPELL_AMPLIFY_DAMAGE);
+                        AmplifyDamageTimer = urand(20000, 30000);
                 }
                 else
                     AmplifyDamageTimer -= diff;