Add missing thumbnail for TermSet tutorial

[oruebel]

Motivation

Fix #1203

Checklist

• Did you update CHANGELOG.md with your changes?
• Does the PR clearly describe the problem and the solution?
• Have you reviewed our Contributing Guide?
• Does the PR use "Fix #XXX" notation to tell GitHub to close the relevant issue numbered XXX when the PR is merged?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.12%. Comparing base (be602e5) to head (bf6e4f3).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1204   +/-   ##
=======================================
  Coverage   89.12%   89.12%           
=======================================
  Files          45       45           
  Lines        9944     9944           
  Branches     2825     2825           
=======================================
  Hits         8863     8863           
  Misses        762      762           
  Partials      319      319           

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