docs: correct mdformat-admon plugin name

[KyleKing]

Fixes #519

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (10307b9) to head (7e07461).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #520   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           15        15           
  Lines          970       970           
  Branches       171       171           
=========================================
  Hits           970       970           

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[hukkin]

Thank you!