message: docs: make code snippet usable with required imports
diff: @@ -207,7 +207,11 @@ CUSTOM_SECURITY_MANAGER = CustomSsoSecurityManager
  the app object and can alter it in any way. For example, add `FLASK_APP_MUTATOR` into your
  `superset_config.py` to setup session cookie expiration time to 24 hours:
  -```
  +```python
  +from flask import session
  +from flask import Flask
  +
  +
  def make_session_permanent():
  '''
  Enable maxAge for the cookie 'session'
  