fmt: fix readme link (IEC-53)

[robertlipe]

Checklist

• [ ] Component contains License
• [ ] Component contains README.md
• [ ] Component contains idf_component.yml file with url field defined
• [ ] Component was added to upload job
• [ ] Component was added to build job
• [ ] Optional: Component contains unit tests
• [ ] CI passing

Change description

"Fix" broken link in {fmt} doc, but I don't feel great about this fix. It's surely overly specific, but I couldn't work out the optimal short-form, relative links either.

[robertlipe]

This works, but it feels bad to reach up and into another project.