fix a typo

auracast/multicast_control.py

@@ -71,7 +71,7 @@ class Multicaster:
         self. is_audio_init = False
         if self.device:
             await self.device.stop_advertising()
-        if self.big:
+        if self.bigs:
             for big in self.bigs.values():
                 if big['advertising_set']:
                     await big['advertising_set'].stop()