minor #5073 [Cookbook] Add note about possible 404 error on Heroku (bicpi)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook] Add note about possible 404 error on Heroku

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

I was running into this 404 and thought something went wrong with the doc root setting first.

Commits
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1af3913 [Cookbook] Add note about possible 404 error

Author: weaverryan

cookbook/deployment/heroku.rst

@@ -220,6 +220,14 @@ application will respond:
 
 You should be seeing your Symfony application in your browser.
 
+.. caution::
+
+    If you take your first steps on Heroku using a fresh installation of
+    the Symfony Standard Edition, you may run into a 404 page not found error.
+    This is because the route for ``/`` is defined by the AcmeDemoBundle, but the
+    AcmeDemoBundle is only loaded in the dev environment (check out your
+    ``AppKernel`` class). Try opening ``/app/example`` from the AppBundle.
+
 .. _`the original article`: https://devcenter.heroku.com/articles/getting-started-with-symfony2
 .. _`signup with Heroku`: https://signup.heroku.com/signup/dc
 .. _`Heroku Toolbelt`: https://devcenter.heroku.com/articles/getting-started-with-php#local-workstation-setup