Style Guide should mention standardize use of Carpentry/Carpentries/The Carpentries

[rgaiacs]

This is related to https://github.com/carpentries/instructor-training/issues/639. http://swcarpentry.github.io/lesson-example/06-style-guide/index.html should mention the "correct" way to mention the organisation. Also, would be good if our validation tool check for it.