Fix inconsistency in the documentation

[walpox]

Sanity check:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

• Are you doing the PR on the next branch?

From CONTRIBUTING.md:

As the documentation site is automatically built on commits to master, all development happens on the next branch, unless it is fixing the current documentation.
However, if you notice an error or typo in the documentation, feel free to directly submit a PR without opening an issue.

Context

The documented way to add new features is inconsistent between the Syntax Highlighting page in the site and the Contribution guidelines in the repo.

[Keats]

Thanks!