docs: Small typo in using_session_auth.md

[obozdag]

Description Explain what you have changed, and why.

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [x] User guide updated
• [ ] Conforms to style guide

[datamweb]

@obozdag thank you!