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

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.

Start with the style guide at http://swcarpentry.github.io/lesson-example/06-style-guide/index.html and review the related instructor-training issue 639 for the organisation naming guidance. Identify the repository's validation tool, then update the guide and ensure the validator checks the stated usage; done means both the guidance and validation behavior cover the agreed wording.

Written by the indexing model from the issue text.