added the example Twig Code for the HTML5 no novalidate attribute

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes, a Twig example was added
| Applies to    | 2.3
| Fixed tickets | no PR

in the form documentation, there is a notice about the no validate HTML5 attribute, but without the twig example.

Symfony 2.3 added the method {{ form(form) }} used in the example