docs: update action handler to be asynchronous

Author: NoOne7135

adminforth/documentation/docs/tutorial/03-Customization/09-Actions.md

@@ -21,7 +21,7 @@ Here's how to add a custom action:
         },
 
         // Handler function when action is triggered
-        action: ({ recordId, adminUser }) => {
+        action: async ({ recordId, adminUser }) => {
           console.log("auto submit", recordId, adminUser);
           return { 
             ok: true,