[DependencyInjection] Fix variable description in ConfigurationExtensionInterface

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10845
| License       | MIT
| Doc PR        | none

This fixes the description of parameter `array $config` in file [ConfigurationExtensionInterface.php](https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/DependencyInjection/Extension/ConfigurationExtensionInterface.php) by removing the extra `$config`.