update copy file snippet error handling

Author: osama-rizk

src/pages/[platform]/build-a-backend/storage/copy-files/index.mdx

@@ -101,7 +101,7 @@ const copyFile = async () => {
       }
     });
   } catch (error) {
-    console.error('Error', err);
+    console.error('Error', error);
   }
 };
 ```