Fix incorrect return type in doc-comment (#410)
Fix incorrect return type in doc-comment (#410)

Method returns a Table not a Dataset. This throws off IDEs and leads users to dive into the documentation to work out why they can't do:

```php
        $table = $dataset->table($tableName);

        $job = $table->load(...)
```
chingor13/google-cloud-php,chingor13/google-cloud-php,pvm1987/google-cloud-php,jdpedrie/google-cloud-php,tmatsuo/google-cloud-php,pvm1987/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,jdpedrie/google-cloud-php,chingor13/google-cloud-php,tmatsuo/google-cloud-php,jdpedrie/google-cloud-php