Update url for RP initiated logout

[charleswhchan]

According to urls.py, the url should be /logout

Checklist

• [ ] PR only contains one change (considered splitting up PR)
• [ ] unit-test added
• [x] documentation updated
• [ ] CHANGELOG.md updated (only for user relevant changes)
• [ ] author name in AUTHORS

[codecov[bot]]

Codecov Report

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

Project coverage is 97.56%. Comparing base (560f84d) to head (bff8e5c). Report is 6 commits behind head on master.

:exclamation: Current head bff8e5c differs from pull request most recent head c3db05f. Consider uploading reports for the commit c3db05f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1405 +/- ## ======================================= Coverage 97.56% 97.56% ======================================= Files 32 32 Lines 2132 2132 ======================================= Hits 2080 2080 Misses 52 52 ```

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