fix(Scripts/UBRS): Rend's summon spell should be a triggered cast (#9074)

Fixes him not being summoned if Gyth's casting something when the transition hits.

src/server/scripts/EasternKingdoms/BlackrockMountain/BlackrockSpire/boss_gyth.cpp

@@ -105,7 +105,7 @@ public:
                     // Let creature fall to 1 HP but prevent it from dying before boss is summoned.
                     damage = me->GetHealth() - 1;
                 }
-                DoCast(me, SPELL_SUMMON_REND);
+                DoCast(me, SPELL_SUMMON_REND, true);
                 me->RemoveAura(SPELL_REND_MOUNTS);
                 _summonedRend = true;
             }