fixed error in fireball due to the boom effect from upgrade (#6)

Reviewed-on: #6
Co-authored-by: 3002102 <3002102@stud.hs-mannheim.de>
Co-committed-by: 3002102 <3002102@stud.hs-mannheim.de>

scripts/fireball.gd

@@ -10,6 +10,6 @@ func _on_body_entered(body: Node2D) -> void:
 		if fireball_aoe:
 			var boom = explosion_scene.instantiate()
 			boom.global_position = global_position
-			get_parent().add_child(boom) 
+			get_parent().call_deferred("add_child", boom)
 		queue_free()