Fix Traefik router name in SRV server delegation docs

The wildcard certificate examples attached tls.domains labels to a
router named matrix-synapse-federation-api, but the Synapse role
actually names it matrix-synapse-public-federation-api (see
templates/synapse/labels.j2), so the documented labels matched nothing
and the certificate configuration silently did not apply.

Fixes #3129

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/howto-srv-server-delegation.md

@@ -49,8 +49,8 @@ This is because the Matrix specification expects the federation endpoint to be s
 ```yaml
 # To let Traefik know which domains' certificates to serve
 matrix_synapse_container_labels_additional_labels: |
-  traefik.http.routers.matrix-synapse-federation-api.tls.domains.main="example.com"
-  traefik.http.routers.matrix-synapse-federation-api.tls.domains.sans="*.example.com"
+  traefik.http.routers.matrix-synapse-public-federation-api.tls.domains.main="example.com"
+  traefik.http.routers.matrix-synapse-public-federation-api.tls.domains.sans="*.example.com"
 ```
 
 ### Configure the DNS-01 challenge for let's encrypt
@@ -137,8 +137,8 @@ matrix_synapse_container_labels_public_federation_api_traefik_rule: PathPrefix(`
 
 # To let Traefik know which domains' certificates to serve
 matrix_synapse_container_labels_additional_labels: |
-  traefik.http.routers.matrix-synapse-federation-api.tls.domains.main="example.com"
-  traefik.http.routers.matrix-synapse-federation-api.tls.domains.sans="*.example.com"
+  traefik.http.routers.matrix-synapse-public-federation-api.tls.domains.main="example.com"
+  traefik.http.routers.matrix-synapse-public-federation-api.tls.domains.sans="*.example.com"
 
 # Add a new ACME configuration without having to disable the default one, since it would have a wide range of side effects
 traefik_configuration_extension_yaml: |