doc: add port to PUBLIC_URL

Fixes #931

env.example

@@ -45,7 +45,7 @@ HTTPS_PORT=8443
 TZ=UTC
 
 # Public URL for the web service (required)
-#PUBLIC_URL=https://meet.example.com
+#PUBLIC_URL=https://meet.example.com:8443
 
 # IP address of the Docker host
 # See the "Running behind NAT or on a LAN environment" section in the Handbook: