docs(reflow/tutorial): drop misleading API key on /health troubleshooting curl
/health is a public endpoint — it does not require X-API-Key.
Including the header in the debugging tip implied otherwise.
Swap to the plain curl plus a parenthetical reminder that the
endpoint is public.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>