Fix the semantic configuration example for the Resource Bundle

Q  | A
----|----
Doc fix? | yes
New docs? | no
Fixed tickets | 

When following the steps to used the [advanced configuration](http://sylius.readthedocs.org/en/latest/bundles/SyliusResourceBundle/configuration.html#advanced-configuration) for my resource bundle, following the example for exposing the semantic configuration of the bundle will produce a fatal error:

    Fatal error: Call to undefined method Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition::scalarNode()

To fix this, the root node should define `children` first:

```php
$rootNode
    ->children()
        ->scalarNode('driver')->isRequired()->cannotBeEmpty()->end()
        // other nodes
    ->end()
;
```