[cookbook][doctrine] fixed reverse_engineering doc

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.1+
| Fixed tickets | N/A

Sorry but I could find no evidence of update from the code. 
I wrote the way that actually works for me (in Symfony SE 2.1/2.2).
If the evidence for this fix is required, I will dig into code... 
Maybe they are in doctrine/DoctrineBundle or doctrine/orm?