minor #28402 [TwigBundle] Using Twig template name syntax in form_theme example (yceruto)

This PR was merged into the 4.2-dev branch.

Discussion
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[TwigBundle] Using Twig template name syntax in form_theme example

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Minor tweak to remove the `symfony/templating` syntax.

Commits
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ce653f274b Using Twig template name syntax in form_theme example

src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php

@@ -54,7 +54,7 @@ class Configuration implements ConfigurationInterface
                 ->arrayNode('form_themes')
                     ->addDefaultChildrenIfNoneSet()
                     ->prototype('scalar')->defaultValue('form_div_layout.html.twig')->end()
-                    ->example(array('MyBundle::form.html.twig'))
+                    ->example(array('@My/form.html.twig'))
                     ->validate()
                         ->ifTrue(function ($v) { return !\in_array('form_div_layout.html.twig', $v); })
                         ->then(function ($v) {