Update extensions tutorial

[bendichter]

Motivation

What was the reasoning behind this change? Please explain the changes briefly.

How to test the behavior?

Show how to reproduce the new behavior (can be a bug fix or a new feature)

Checklist

• [ ] Did you update CHANGELOG.md with your changes?
• [ ] Have you checked our Contributing document?
• [ ] Have you ensured the PR clearly describes the problem and the solution?
• [ ] Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• [ ] Have you checked to ensure that there aren't other open Pull Requests for the same change?
• [ ] Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.