!15655 modify demo code

Merge pull request !15655 from 胡伟奇/fix_vod

zh-cn/application-dev/file-management/medialibrary-filepath-guidelines.md

@@ -135,7 +135,7 @@ async function copySandbox2Public() {
     console.error('file asset get failed, message = ' + err);
   }
   let fdPub = await fileAsset.open('rw');
-  let fdSand = await fs.open(sandboxDirPath + 'testFile.txt', OpenMode.READ_WRITE);
+  let fdSand = await fs.open(sandboxDirPath + 'testFile.txt', fs.OpenMode.READ_WRITE);
   await fs.copyFile(fdSand.fd, fdPub);
   await fileAsset.close(fdPub);
   await fs.close(fdSand.fd);