fmt: fix link to parent project in README.md (IEC-145)

[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

An embarrassingly simple "fix". The correct link is

https://github.com/fmtlib/fmt/blob/master/README.md not https://github.com/fmtlib/fmt/blob/master/README.rst

...and apparently has been since fmt was uploaded to Github in 2012.

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