nothing example misuses type annotations
Summary: The order-based dependencies example we provide would not pass mypy, because it uses a Nothing type annotation, but the solid doesn't return a value of type Nothing. The example works fine without the annotation, so I just took it out.
Test Plan: bk
Reviewers: cdecarolis, owen, alangenfeld
Reviewed By: owen
Differential Revision: https://dagster.phacility.com/D8946