[book] Avoid scrollbars in code examples

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

There is one thing I'm not sure about and that thing are the wrapped commands:

````
$ php app/console generate:bundle AcmeFooBundle \
--dir src --format yaml
````

I don't know if it is commonly known that the `\` character can be used for multiline commands