[Hackday] Update Heroku doc with SYMFONY_ENV param

| Q             | A
| ------------- | ---
| Doc fix?      | [yes]
| New docs?     | [no]
| Applies to    | [2.5, 2.6, 2.7]
| Fixed tickets |  #4217
@timglabisch its relates only to Heroku, without this environment variable your app will fall with a error on pushing to heroku master step. Many developers have faced this problem and if this will be written in official documentation it can help to save a lot of time.