fix typo in Session Middleware docs

[derenrich]

Summary

Just fixing a docs typo

Checklist

• [x ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x ] I've updated the documentation accordingly.

[Kludex]

Thanks @derenrich :)