Fixed syntax error

app.py

@@ -7,7 +7,7 @@ class State:
 
 state = State()
 
-window.onMessage = event => alert('Test message received: ' + event.data)
+window.onMessage = (event) => {alert('Test message received: ' + event.data)}
 
 def update_values(context, question):
     output = context + "\n" + question