Update supervisor config in getting started guide

[amclain]

Description

Update the example code in the getting started guide for starting Scenic in a supervision tree.

Motivation and Context

When I used the code in the getting started guide for a recent project, the example code no longer worked.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.

[axelson]

Ah nice!