Fix example with wrong handler parameter name

Author: ileacristian

examples/pub_sub_more_on_filter.py

@@ -31,7 +31,7 @@ def send_messages(agent):
     time.sleep(2)
 
     bob.unsubscribe('listener', 'a')
-    bob.subscribe('listener', handlers={'b': log_b})
+    bob.subscribe('listener', handler={'b': log_b})
 
     time.sleep(2)