Bugfix: Fix wrong use call of external_multiple_structure

classes/external/get_submission_mobile.php

@@ -27,7 +27,7 @@
 namespace mod_booking\external;
 
 use cache;
-use core_external\external_multiple_structure;
+use external_multiple_structure;
 use core_form\external\dynamic_form;
 use external_api;
 use external_function_parameters;