minor #10549 Update external_parameters.rst (ddegasperi)

This PR was merged into the 4.1 branch.

Discussion
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Update external_parameters.rst

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Commits
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7042e33 Update external_parameters.rst

Author: javiereguiluz

configuration/external_parameters.rst

@@ -474,15 +474,10 @@ create a class that implements
 :class:`Symfony\\Component\\DependencyInjection\\EnvVarProcessorInterface` and
 then, define a service for that class::
 
+    use Symfony\Component\DependencyInjection\EnvVarProcessorInterface;
+
     class LowercasingEnvVarProcessor implements EnvVarProcessorInterface
     {
-        private $container;
-
-        public function __construct(ContainerInterface $container)
-        {
-            $this->container = $container;
-        }
-
         public function getEnv($prefix, $name, \Closure $getEnv)
         {
             $env = $getEnv($name);