Scripts/Tempest Keep: Fixed a typo in Nethermancer Sepethrea

Closes #23828
Ty Rizztazz

(cherry picked from commit 746a1d89b34984f31ab66ef00e93a648dc42af5d)

src/server/scripts/Outland/TempestKeep/Mechanar/boss_nethermancer_sepethrea.cpp

@@ -192,7 +192,7 @@ class npc_ragin_flames : public CreatureScript
                     //Check_Timer
                     if (Check_Timer <= diff)
                     {
-                        if (instance->GetData(DATA_NETHERMANCER_SEPRETHREA) != IN_PROGRESS)
+                        if (instance->GetBossState(DATA_NETHERMANCER_SEPRETHREA) != IN_PROGRESS)
                         {
                             //remove
                             me->DespawnOrUnsummon();