fix: add missing arrow operator in onProgress callback function (#8379)

* fix: add missing arrow operator in onProgress callback function

* update copy file snippet error handling

* revert dart syntax change

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);
   }
 };
 ```

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

@@ -707,7 +707,7 @@ const { body, eTag } = await downloadData(
   {
     path: 'album/2024/1.jpg',
     options: {
-      onProgress: (progress) {
+      onProgress: (progress) => {
         console.log(`Download progress: ${(progress.transferredBytes/progress.totalBytes) * 100}%`);
       }
     }