docs(external-tls): generalize certbot contact email to <domain> placeholder

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

external-tls.md

@@ -30,7 +30,7 @@ Keys are **ECDSA** (`--key-type ecdsa`), matching the rest of the fleet.
 
 ```sh
 sudo certbot certonly \
-    -m ops@zap.pics --agree-tos --no-eff-email --noninteractive \
+    -m ops@<domain> --agree-tos --no-eff-email --noninteractive \
     --cert-name <domain> \
     --key-type ecdsa \
     --dns-cloudflare \