[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
is it really a good idea to show the complete content of the composer.json here?
this will become bad if we have to maintain this for different versions. i would prefer something like:

make sure you removed: ....
make sure you added: ....