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