Update main_concepts.rst

[glad4enkonm]

custom link fix

This is a 🔦 documentation change.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.90%. Comparing base (4ea3547) to head (8e513ee).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1275   +/-   ##
=======================================
  Coverage   79.90%   79.90%           
=======================================
  Files         146      146           
  Lines       10031    10031           
=======================================
  Hits         8015     8015           
  Misses       2016     2016           

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