Webhook Url verification is returning with additional string

I am trying to verify my webhook url, however I am getting
Here is my code:

```
$config = [

'facebook' => [
'token' => 'page-token',
'app_secret' => 'app-secret',
'verification' => env('VERIFY_TOKEN')
]

];

DriverManager::loadDriver(FacebookDriver::class);

$botman = BotManFactory::create($config);

$botman->listen();
```
As noticed aboved I am getting additonal string added on the hub_challenge request. How will I able to solved this problem?

Reproduce the webhook verification request using the PHP configuration shown and inspect FacebookDriver's handling of the hub_challenge response. Confirm the expected response format and identify why an additional string is returned; done means the verification response contains only the required challenge value.

Written by the indexing model from the issue text.