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.

[codecov]

Codecov Report

Merging #1661 (5d3d12a) into dev (7354813) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1661   +/-   ##
=======================================
  Coverage   91.99%   91.99%           
=======================================
  Files          26       26           
  Lines        2585     2585           
  Branches      496      496           
=======================================
  Hits         2378     2378           
  Misses        132      132           
  Partials       75       75           

Flag	Coverage Δ
integration	71.52% <ø> (ø)
unit	83.67% <ø> (ø)

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