Use |regex_escape in Whatsapp registration.yaml

Doesn't matter much, but it makes it consistent with the other bridges.

roles/matrix-bridge-mautrix-whatsapp/defaults/main.yml

@@ -185,7 +185,7 @@ matrix_mautrix_whatsapp_registration_yaml: |
   rate_limited: false
   namespaces:
       users:
-      - regex: '^@whatsapp_[0-9]+:{{ matrix_mautrix_whatsapp_homeserver_domain }}$'
+      - regex: '^@whatsapp_[0-9]+:{{ matrix_mautrix_whatsapp_homeserver_domain|regex_escape }}$'
         exclusive: true
 
 matrix_mautrix_whatsapp_registration: "{{ matrix_mautrix_whatsapp_registration_yaml|from_yaml }}"