4.0->master: Updated link+title of Heroku's doc

* Updated link for branches 4.0 to 4.2 (remove doc link for Symfony 3)
* Changed title (especially for branches 5.0 to master) : the documentation also talks about Symfony 5

Author: Pyrrah

deployment/heroku.rst

@@ -7,6 +7,6 @@ Deploying to Heroku
 ===================
 
 To deploy to Heroku, see their official documentation:
-`Deploying Symfony 4 Apps on Heroku`_.
+`Deploying Symfony 4 & 5 Apps on Heroku`_.
 
-.. _`Deploying Symfony 4 Apps on Heroku`: https://devcenter.heroku.com/articles/deploying-symfony4
+.. _`Deploying Symfony 4 & 5 Apps on Heroku`: https://devcenter.heroku.com/articles/deploying-symfony4