Update third_party-portainer.md

docs/third_party-portainer.md

@@ -2,6 +2,8 @@ In order to enable Portainer, the docker-compose.yml and site.conf for Nginx mus
 
 1\. Create a new file `docker-compose.override.yml` in the mailcow-dockerized root folder and insert the following configuration
 ```
+version: '2.1'
+services:
     portainer-mailcow:
       image: portainer/portainer
       volumes: