[WebBundle] fixed typo

src/Symfony/Framework/WebBundle/Resources/config/pdo.xml

@@ -5,7 +5,7 @@
     xsi:schemaLocation="http://www.symfony-project.org/schema/dic/services http://www.symfony-project.org/schema/dic/services/services-1.0.xsd">
 
   <service id="pdo_connection" class="PDO">
-    <argument key="dns">%pdo.dsn%</argument>
+    <argument key="dsn">%pdo.dsn%</argument>
     <argument key="username">%pdo.username%</argument>
     <argument key="password">%pdo.password%</argument>
   </service>