Subclass Sandbox Markdown file edits

This pull requests includes proofreading edits to the Subclass Sandbox Markdown file.

Note that the only style issue I found was that the *Superpower* class was italicized in "We do this by making these operations *protected methods of the `Superpower` base class*." I changed the italics so that it reads, "We do this by making these operations *protected methods of the* `Superpower` *base class*." Then, I realized that on your website, `Superpower` is not italicized in that sentence, so I suppose your Markdown-to-HTML converter fixes this! I left the change anyway in case you end up using a Markdown-to-MOBI/EPUB/etc. converter when publishing your book.