minor #5173 Use $this->getRootDir() instead of __DIR__ (manelselles)

This PR was merged into the 2.3 branch.

Discussion
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Use $this->getRootDir() instead of __DIR__

Use $this->getRootDir() when loading the configuration file from AppKernel.

Symfony standard version uses $this->getRootDir() so we should have the same version on both sites.

Commits
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e0437ea Use $this->getRootDir() instead of __DIR__

cookbook/configuration/environments.rst

@@ -45,7 +45,7 @@ class:
 
         public function registerContainerConfiguration(LoaderInterface $loader)
         {
-            $loader->load(__DIR__.'/config/config_'.$this->getEnvironment().'.yml');
+            $loader->load($this->getRootDir().'/config/config_'.$this->getEnvironment().'.yml');
         }
     }