Update config_override_message.services.yml: change '@app.root' to '%app.root%'

Closes #3393755

config_override_message.services.yml

 services:
   config_override_message.manager:
     class: Drupal\config_override_message\ConfigOverrideMessageManager
-    arguments: ['@app.root', '@module_handler', '@cache.data', '@file_system']
+    arguments: ['%app.root%', '@module_handler', '@cache.data', '@file_system']
 
   config_override_message.subscriber:
     class: Drupal\config_override_message\EventSubscriber\ConfigOverrideMessageSubscriber