Clarify tip for creating a new AppBundle

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

The tip seems to be confusing, since the AppBundle is also generated with `symfony new blog 2.3.23` (and 2.5.8). In addition the mentioned `generate:bundle` command does not work in SF 2.3 SE, since it stops with a `The namespace must contain a vendor namespace` error.
What do you think of "If your Symfony installation doesn't come with a pregenerated `AppBundle`, you can generate it by hand executing this command:"?