Update mta-sts.mdx (#21181)

The template mentioned in the doc no longer exists.

Author: cf-scott

src/content/docs/email-routing/setup/mta-sts.mdx

@@ -31,9 +31,15 @@ Next you need an HTTPS endpoint at `mta-sts.example.com` to serve your policy fi
 
 To do this you need to deploy a Worker that allows email clients to pull Cloudflare’s Email Routing policy file using the “well-known” URI convention.
 
-4. Go to your **Account** > **Workers & Pages** and press **Create Application**. Pick the "MTA-STS" template from the list.
-
-![MTA-STS Worker](~/assets/images/email-routing/mta-sts-worker.png)
+4. Go to your **Account** > **Workers & Pages** and press **Create Application**. Pick the default "Hello World" option button, and replace the sample worker code with the following:
+
+```index.js
+export default {
+  async fetch(request, env, ctx) {
+    return await fetch("https://mta-sts.mx.cloudflare.net/.well-known/mta-sts.txt")
+  },
+};
+```
 
 5. This Worker proxies `https://mta-sts.mx.cloudflare.net/.well-known/mta-sts.txt` to your own domain. After deploying it, go to the Worker configuration, then **Triggers** > **Custom Domains** and **Add Custom Domain**. Type the subdomain `mta-sts.example.com`.