Fix link of phpunit/phpunit example

[eliashaeussler]

This pull request

• fixes the link of phpunit/phpunit example in README.md.

[codecov]

Codecov Report

Merging #924 (4e50535) into main (0045fae) will not change coverage.
The diff coverage is n/a.

❗ Current head 4e50535 differs from pull request most recent head 005d6cf. Consider uploading reports for the commit 005d6cf to get more accurate results

@@            Coverage Diff            @@
##               main     #924   +/-   ##
=========================================
  Coverage     88.82%   88.82%           
  Complexity       55       55           
=========================================
  Files             4        4           
  Lines           188      188           
=========================================
  Hits            167      167           
  Misses           21       21           

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[localheinz]

Thank you, @eliashaeussler!