Correct typo in docstring

Author: brefra

plugwise/controller.py

@@ -63,7 +63,7 @@ def connect_to_stick(self, callback=None) -> bool:
         """
         Connect to USB-Stick and startup all worker threads
 
-        Return: True when connection is successfull.
+        Return: True when connection is successful.
         """
         self.init_callback = callback
         # Open connection to USB Stick