The $link argument must be passed as a reference

As the title says the `$link` argument in `close_database_connection` function definition must be accepted as a reference like `close_database_connection(&$link)`.

Author: Muhammad Nasir Rahimi

book/from_flat_php_to_symfony2.rst

@@ -148,7 +148,7 @@ of the application are isolated in a new file called ``model.php``::
         return $link;
     }
 
-    function close_database_connection($link)
+    function close_database_connection(&$link)
     {
         $link = null;
     }