[TheBook][Testing] - Fixed example, wrong access to header property on Client object

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | NA


In the example on how to validate headers $client tries to access Symfony\Bundle\FrameworkBundle\Client::headers directly this property is protected. The correct way is Symfony\Bundle\FrameworkBundle\Client::getHeaders()