Broken Link - Contribution.md

Getting a 404 error on the "Content" link, from "The [Content](https://brand.github.com/content/) section is a great place to start." under the Styling Content heading on the [CONTRIBUTION.MD page](https://github.com/github/training-kit/blob/main/CONTRIBUTING.md)

Open CONTRIBUTING.md and find the Styling Content heading and its Content link. Check the referenced destination and update the broken link so the documentation points to a working page, then verify the link no longer returns a 404.

Written by the indexing model from the issue text.