Fix docs about heartbeat status on warnings (200, not 5XX)

[leplatrem]

All implementation follow this logic:

if check_results.level < checks.ERROR:
        status_code = 200        
else:
        status_code = 500

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.97%. Comparing base (2ba0254) to head (f670e83).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   97.97%   97.97%           
=======================================
  Files          22       22           
  Lines         692      692           
  Branches       92       92           
=======================================
  Hits          678      678           
  Misses          8        8           
  Partials        6        6           

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