Pull Request: carpentries/lesson-example#225

* replace sub-heading with sub-title

Pull Request: carpentries/lesson-example#237

fix typo

As suggested by @gcapes.

that is missing formating.

Close swcarpentry/lesson-example#195

This information is being migrated to the Carpentries Handbook:

- [Maintainer Onboarding Process](http://docs.carpentries.org/topic_folders/maintainers/maintainers.html#maintainer-onboarding)
- [Lesson Release Process and Schedule](http://docs.carpentries.org/topic_folders/lesson_development/lesson_release.html)
- [GitHub Issue Tags](http://docs.carpentries.org/topic_folders/maintainers/github_labels.html)

The 'input' (raw) block showed the `.source` class identifier, but the rendered output used `.language-python`.

Add HTML indenting.

Close #134

Close #129

Thanks to @fmichonneau, @clintonroy and @willingc.