Fix Discord server link in contributing guide

[fredbi]

Updated Discord invite link in CONTRIBUTING.md

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.29%. Comparing base (0ffc18b) to head (a422afb).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #71   +/-   ##
=======================================
  Coverage   91.28%   91.29%           
=======================================
  Files          91       91           
  Lines       11925    11925           
=======================================
+ Hits        10886    10887    +1     
+ Misses        826      824    -2     
- Partials      213      214    +1     

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