docs: Fixed URL in CONTRIBUTING

[frgfm]

This PR fixes a broken URL in CONTRIBUTING.

Any feedback is welcome!

[codecov]

Codecov Report

Merging #141 (2c3312d) into master (48262ca) will not change coverage.
The diff coverage is n/a.

❗ Current head 2c3312d differs from pull request most recent head 361d158. Consider uploading reports for the commit 361d158 to get more accurate results

@@           Coverage Diff           @@
##           master     #141   +/-   ##
=======================================
  Coverage   89.16%   89.16%           
=======================================
  Files           8        8           
  Lines         203      203           
=======================================
  Hits          181      181           
  Misses         22       22           

Flag	Coverage Δ
unittests	89.16% <ø> (ø)

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[frgfm]

The CI job is doing weird things: the two other links mention do work. They're redirecting to something else and if I change it to the end URL, it still fails 🤷‍♂️

So I'll leave it as is