Update links in PRs section of CONTRIBUTING.md

[usulpro]

CONTRIBUTING.md -> PRs section:

Issue: some of the links point to the labels which no longer exist

If our labels became more or less stable then let's update them

What I did

update links

How to test

check out new links.

[codecov]

Codecov Report

Merging #992 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #992   +/-   ##
======================================
  Coverage    12.6%   12.6%           
======================================
  Files         192     192           
  Lines        4427    4427           
  Branches      707     707           
======================================
  Hits          558     558           
  Misses       3244    3244           
  Partials      625     625

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[shilman]

Great!!

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