Fix markup in "Advanced Usage" doc

[multani]

• Fix YAML indentiation in a configuration example
• Improve Markdown markup

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.

[micahellison]

Thank you for submitting this, and for your attention to detail. All of these changes look great.