Fix api_key docstring in async case too

[pquentin]

I noticed this because https://github.com/elastic/elasticsearch-py/pull/2475 was reverting the change. In fact, the sync code is generated by running unasync on the async code.

[github-actions[bot]]

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.}