Added missing @internal scope to plugin modules doc

Signed-off-by: Andre Wanlin <awanlin@spotify.com>

docs/backend-system/architecture/06-modules.md

@@ -52,7 +52,7 @@ This allows you to install the module in your backend instance by just referenci
 
 ```ts
 backend.add(
-  import('backstage-plugin-catalog-backend-module-example-processor'),
+  import('@internal/backstage-plugin-catalog-backend-module-example-processor'),
 );
 ```