minor symfony#11286 Update project directory method override (tdutrion)

This PR was merged into the 4.2 branch.

Discussion
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Update project directory method override

This follows up symfony/recipes#564 (not yet merged), and updates the documentation according to the actual code provided in the recipe.

Commits
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f48dd00 Update project directory method override

Author: javiereguiluz

reference/configuration/kernel.rst

@@ -81,9 +81,9 @@ method to return the right project directory::
     {
         // ...
 
-        public function getProjectDir()
+        public function getProjectDir(): string
         {
-            return realpath(__DIR__.'/../');
+            return \dirname(__DIR__);
         }
     }