add missing import (#1955)

Author: ksuess

docs/developer-guide/create-a-distribution.md

@@ -90,6 +90,8 @@ The above example registers a distribution that will configure a personal blog w
 When registering a distribution, you can provide a `pre_handler`, a `handler`, and a `post_handler`, each of which must be a function with their respective signature, as shown in the following example.
 
 ```python
+from plone.distribution.core import Distribution
+
 def pre_handler(answers: dict) -> dict:
     return answers