[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
this should be: Build Priority in Composer (all words should be capitialized in headings, except from closed class words)