Use the current template reference in the configuration example

[Amoifr]

Closes #504.

The section says the block lists the default configuration, but the template it shows is neither the default nor resolvable.

Configuration.php already defaults to @KnpMenu/menu.html.twig, and KnpMenuExtensionTest asserts it:

->scalarNode('template')->defaultValue('@KnpMenu/menu.html.twig')->end()

The documented KnpMenuBundle::menu.html.twig is wrong twice over. The Bundle::template syntax was deprecated in Symfony 4.4 and removed in 5.0, so this has been broken for longer than the report suggests, and even under the old syntax it resolved to Resources/views/, while the template has since moved to templates/menu.html.twig.

@vpassama reported the YAML block. The same value appears in the XML and PHP blocks of the same configuration-block, so all three are updated. The YAML one needs quotes, since an unquoted @ starts a reserved indicator and the file would no longer parse.

I left the rest of the section alone, including the loadFromExtension() style of the PHP block, to keep this to the reported problem.