[FrameworkBundle] Fixed typo in traces.html.twig

src/Symfony/Bundle/FrameworkBundle/Resources/views/Exception/traces.html.twig

@@ -11,7 +11,7 @@
             {% endspaceless %}
         </h2>
     {% else %}
-        <h2>Stack Trace</h3>
+        <h2>Stack Trace</h2>
     {% endif %}
 
     <a id="traces_link_{{ position }}"></a>