Fixed file path

"app/Resources/views/Form/fields.html.twig" didn't work for me - see next code block: "form_theme form 'AppBundle:Form:fields.html.twig'"

If this change is correct, I guess it needs to be corrected multiple times on this page.

cookbook/form/form_customization.rst

@@ -282,7 +282,7 @@ can now re-use the form customization across many templates:
 
 .. code-block:: html+twig
 
-    {# app/Resources/views/Form/fields.html.twig #}
+    {# src/AppBundle/Resources/views/Form/fields.html.twig #}
     {% block integer_widget %}
         <div class="integer_widget">
             {% set type = type|default('number') %}