YAML docs contain outdated language

http://symfony.com/doc/current/components/yaml/introduction.html

In the 3.0 version of the docs, there is a hint that reads

```
Because it is currently possible to pass a filename to this method, you must validate the input first. Passing a filename is deprecated in Symfony 2.2, and will be removed in Symfony 3.0.
```

The language here is outdated and could do with re-writing.
