Added doc for group_remote_id

Author: mnocon

docs/users/user_registration.md

@@ -31,7 +31,7 @@ ibexa:
     system:
         default:
             user_registration:
-                group_id: <userGroupContentId>
+                group_remote_id: <userGroupContentRemoteId>
 ```
 
 ## Registration form field configuration
@@ -92,4 +92,4 @@ ibexa.site_access.config.<siteaccess>.user_settings.templates.update
 
 **Changing registration form templates:**
 
-To change the registration form template, follow instructions in [Invitation and registration form templates](invitations.md#invitation-and-registration-form-templates).
+To change the registration form template, follow instructions in [Invitation and registration form templates](invitations.md#invitation-and-registration-form-templates).