Fix the example about overriding the getProjectDir method

This method must return the canonical path for things to work fine everywhere.

Author: stof

Diff for: deployment.rst

@@ -231,7 +231,7 @@ kernel and return your project's root directory::
 
         public function getProjectDir()
         {
-            return __DIR__.'/..';
+            return dirname(__DIR__);
         }
     }