The book: inconsistency in classname used across code examples

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all (or 2.0+)
| Fixed tickets | #2824

Please see "The Book" > Chapter: "HTTP Fundamental" > Section: "A Symfony Request in Action".

The YAML example contains a class named `AcmeDemoBundle`, however the XML and PHP examples refer to a class named `AcmeBlogBundle`.

This pull request addresses this inconsistency.