Remove instacode.de link from README

[alexandear]

Because it's no longer active:

[HeavyWombat]

Thank you very much for noticing and providing the pull request to change it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.19%. Comparing base (0405da3) to head (3495e20).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #247   +/-   ##
=======================================
  Coverage   78.19%   78.19%           
=======================================
  Files           1        1           
  Lines         188      188           
=======================================
  Hits          147      147           
  Misses         37       37           
  Partials        4        4           

Flag	Coverage Δ
unittests	78.19% <ø> (ø)

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