Updated installation instructions to use the new Symfony Installer

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | -
@stof what we're really going to do is to remove from the best practices book all the explanations regarding how to install Symfony and create the application. Soon we'll change the current *"Use Composer to install Symfony"* practice for *"Use the Symfony Installer to create Symfony apps"*. We won't explain anything else because the book explains all in detail.

But while we change that, this PR is a quick fix to the problem of not explaining correctly how to install Symfony. That's why I'd love to see it merged soon :)