Update a broken link to the format-api example

[kkoppenhaver]

What?

As identified in WordPress/Documentation-Issue-Tracker#408, there was a broken link on the Formatting Toolbar API page.

Why?

The link taking users to the gutenberg-examples repo should actually take them to the correct location instead of 404ing.

How?

Updates an outdated GitHub link with the current working link.

Testing Instructions

1. Load the documentation page for the Formatting Toolbar API.
2. Follow the link at the very bottom of the page that prompts users to "Download the format-api example from the gutenberg-examples repository."
3. See that the updated URL for this link directs users to the correct portion of the GitHub repo.

[github-actions]

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[amustaque97]

Thank you for your contribution. Changes look good to me. 🚀

[talldan]

There are failing tests, but they were fixed by #44052, so I'll move ahead with merging.