Fixed YAML in example configuration

[romaricdrigon]

Hello,

This PR fixes a small typo: in YAML, backslashes should be escaped.
Copy-pasting example configuration would crash the application with Symfony >= 3 (and a deprecation warning with Symfony 2.8).

As far I know, it does not break BC (since escaping \ is part of YAML specification).

[npotier]

Indeed, this deprecation is well documented here : https://symfony.com/blog/new-in-symfony-2-8-yaml-deprecations

Thank you @romaricdrigon. it sounds good to me

[romainguerrero]

Thanks @romaricdrigon . May you rebase your branch on the master one to get a modification on the travis setup ?

[romainguerrero]

Well, I merge it as it concern only the documentation