fix plugin documentation of plugin configuration for endSessionEndpoint

[sstoffregen]

fix plugin documentation of plugin configuration for "endSessionEndpoint" accordingly to #402

Testing done

Only a change inside documentation/readme, which cant. be tested.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.04%. Comparing base (40757e7) to head (f1ae8b3).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #439   +/-   ##
=========================================
  Coverage     72.04%   72.04%           
  Complexity      221      221           
=========================================
  Files            17       17           
  Lines          1023     1023           
  Branches        144      144           
=========================================
  Hits            737      737           
  Misses          198      198           
  Partials         88       88           

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[jtnord]

Thanks!