Add instruction for pulling + starting portainer container

docs/third_party-portainer.md

@@ -1,4 +1,4 @@
-In order to enable Portainer, the docker-compose.yml and site.conf for nginx must be modified.
+In order to enable Portainer, the docker-compose.yml and site.conf for Nginx must be modified.
 
 1\. docker-compose.yml: Insert this block for portainer
 ```
@@ -51,4 +51,8 @@ map $http_upgrade $connection_upgrade {
   }
 ```
 
+3\. Then you need to pull the container and restart Nginx:
+```
+docker-compose up -d && docker-compose restart nginx-mailcow
+```
 Now you can simply navigate to https://${MAILCOW_HOSTNAME}/portainer/ to view your Portainer container monitoring page. You’ll then be prompted to specify a new password for the **admin** account. After specifying your password, you’ll then be able to connect to the Portainer UI.