[cookbook][form] Fixing another typo per @stof

Author: weaverryan

cookbook/form/twig_form_customization.rst

@@ -320,7 +320,7 @@ template and then importing it inside your application configuration:
  <!-- app/config/config.xml -->
  <twig:config ...>
  <twig:form>
- <resource>AcmeDemoBundle:Form:fields.html.twig</resource>
+ <twig:resource>AcmeDemoBundle:Form:fields.html.twig</twig:resource>
  </twig:form>
  <!-- ... -->
  </twig:config>