Add documentation around how titlify is smarter titleize

This PR adds doc fixes. Firstly, it adds syntax highlighting to the two code blocks (which I think makes it easier to grok, and it looks a little nicer). But more importantly, it adds the `ActiveSupport::Inflector.titlecase` example to illustrate `titlify`'s superiority. You mention that it's better in the preceding paragraph, but I think an illustration makes it much clearer. (It also helped to dig into the Rails source and look at exactly what `titlecase` was doing, which, sure enough, was just calling `capitalize` on the first character of every string. I was surprised.)

It also pedantically removes a few whitespace errors.