fix: max chars arg missing

main.py

         text = 'Thanks for adding me to "%s"!' % event['space']['displayName']
         return json.jsonify({'text': text})
     elif event['type'] == 'MESSAGE':
-        text = text_model.make_short_sentence()
+        text = text_model.make_short_sentence(280)
         return json.jsonify({
                             'cards': [{
                                 'sections': [{