Fixes timeout to match comment and docs (GoogleCloudPlatform#3139)

Co-authored-by: Leah E. Cole <[email protected]>

Author: gguuss

iot/api-client/mqtt_example/cloudiot_mqtt_example.py

@@ -66,7 +66,7 @@ def create_jwt(project_id, private_key_file, algorithm):
             # The time that the token was issued at
             'iat': datetime.datetime.utcnow(),
             # The time the token expires.
-            'exp': datetime.datetime.utcnow() + datetime.timedelta(minutes=60),
+            'exp': datetime.datetime.utcnow() + datetime.timedelta(minutes=20),
             # The audience field should always be set to the GCP project id.
             'aud': project_id
     }