Fix simulate client docs

[pquentin]

I failed to notice this in #2737. To avoid this in the future, I've enabled fail_on_warning in Read the Docs: https://docs.readthedocs.io/en/stable/config-file/v2.html#sphinx-fail-on-warning

[github-actions]

A documentation preview will be available soon.

• 🔨 Buildkite builds
• 📚 HTML diff
• 📙 Preview page

Request a new doc build by commenting

• Rebuild this PR: run docs-build
• Rebuild this PR and all Elastic docs: run docs-build rebuild

_{run docs-build is much faster than run docs-build rebuild. A rebuild should only be needed in rare situations.}

_{If your PR continues to fail for an unknown reason, the doc build pipeline may be broken. Elastic employees can check the pipeline status here.}