Fix curl command to properly show headers

Author: ngayerie

src/content/docs/speed/troubleshooting/slow-website.mdx

@@ -24,7 +24,7 @@ Before troubleshooting performance, confirm that your traffic is actually going
 Every response served through Cloudflare includes a `cf-ray` header. Use `curl` to check:
 
 ```bash
-curl -sI https://www.example.com | grep -i cf-ray
+curl -s -D- -o /dev/null https://www.example.com | grep -i cf-ray
 ```
 
 If you see a `cf-ray` header (for example, `cf-ray: 8a1b2c3d4e5f6g7h-SJC`), your traffic is going through Cloudflare. If not, check your DNS configuration.