Update README.md

Fix docker compose up warning message 

```
WARN[0000] network default: network.external.name is deprecated in favor of network.name
```

docs/advanced-config/README.md

@@ -18,7 +18,7 @@ services running on this Docker host:
 ```yml
 networks:
   default:
-    external:
+    external: true
     name: scoobydoo
 ```
 
@@ -38,7 +38,7 @@ services:
 
 networks:
   default:
-    external:
+    external: true
     name: scoobydoo
 ```