Add yaml example in documentation

The title says everythings ;)
Symfoy2's parser is based on the 1.1 Yaml Specification. This allows you to write it without the quotes.

On symfony.com however, the Yaml highlighter is based on the 1.0 Yaml Specification. Which has declared `@` at the start of a scalar as a reserved indicator. Thus you will always see strings starting with `@` in quotes in the documentation. But it is no longer required.

It can be escaped by both single and double quotes. However, double quotes are more commonly used and it is also what you get when dumping it with the Symfony2 Yaml Component.