Minor fix in stop method, use correct property to check if the client is connected

Author: dhoomakethu

pymodbus/client/async/asyncio/__init__.py

@@ -687,7 +687,7 @@ def stop(self):
         Stops connection
         :return:
         """
-        if self._connected.is_set():
+        if self._connected:
             if self.protocol:
                 if self.protocol.transport:
                     self.protocol.transport.close()