Update heroku.md

Without composer symfony/apache-pack, you cannot access any route of your app in production.

Author: younesdiouri

deployment/heroku.md

@@ -57,7 +57,10 @@ Create a new file named `Procfile` in the `api/` directory with the following co
 ```yaml
 web: vendor/bin/heroku-php-apache2 public/
 ```
-
+Be sure to add the symfony/apache-pack in your dependencies : 
+```php
+composer require symfony/apache-pack
+```
 As Heroku doesn't support Varnish out of the box, let's remove it's integration:
 
 ```patch