doc: fix installation instruction link CONTRIBUTING.md

[Lattay]

Just documenting the steps up to having pytest running.

[ml-evs]

Thanks @Lattay, I think I will commit my suggested changes and merge this (essentially just fixing the broken link that you pointed out).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.28%. Comparing base (bc3439a) to head (cdbdd7e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2415   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          78       78           
  Lines        5086     5086           
=======================================
  Hits         4592     4592           
  Misses        494      494           

Flag	Coverage Δ
project	90.28% <ø> (ø)
validator	90.28% <ø> (ø)

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