Update knn declaration in cluster.h

[IlliaOvcharenko]

fix #235

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.28%. Comparing base (f2d9919) to head (e3a7757).
Report is 7 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
- Coverage   97.32%   97.28%   -0.04%     
==========================================
  Files          11       11              
  Lines         224      221       -3     
==========================================
- Hits          218      215       -3     
  Misses          6        6              

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