fix plugin documentation of plugin configuration for `endSessionEndpoint`

[sstoffregen]

fix plugin documentation of plugin configuration for "endSessionEndpoint" accordingly to https://github.com/jenkinsci/oic-auth-plugin/pull/402

Testing done

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

Submitter checklist

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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