Misleading Sample/Documentation (samples/downloadFolderWithTransferManager.js)

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### Documentation Request

# Fix misleading documentation in downloadFolderWithTransferManager.js sample

The documentation comment on line 31 of `samples/downloadFolderWithTransferManager.js` is misleading:

```javascript
// The ID of the GCS folder to download. The folder will be downloaded to the local path of the executing code.
```

However, the current implementation:
```javascript
await transferManager.downloadManyFiles(folderName);
```
Actually returns a Buffer[] array rather than downloading files to the local filesystem as the comment suggests.

To actually download files to the local filesystem as documented, the sample should include the passthroughOptions.destination parameter:

```
await transferManager.downloadManyFiles(folderName, {
passthroughOptions: {
destination: `${__dirname}/`
}
});
```
This is correctly noted on the [Transfer Manager Reference](https://cloud.google.com/nodejs/docs/reference/storage/latest/storage/transfermanager)
which was how I was able to find out how I was supposed to write to the file system.

Either update the code to include the destination option, or modify the comment to accurately reflect that the current implementation returns buffers rather than writing to disk.