minor #12080 Update heroku official documentation link (romainnorberg)

This PR was merged into the 4.3 branch.

Discussion
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Update heroku official documentation link

update the link to point directly to the documentation of Symfony 4 _(and not version 3)_

Actual page: https://symfony.com/doc/current/deployment/heroku.html

Commits
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27c8da8 Update heroku link

Author: OskarStark

deployment/heroku.rst

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