minor #7155 Updated the Heroku deployment article (javiereguiluz)

This PR was merged into the 3.1 branch.

Discussion
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Updated the Heroku deployment article

This finishes #6750 with the changes proposed by @weaverryan. I'm pinging @dzuelke to kindly ask him to review of these changes before merging them. Thanks!

Commits
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815bb81 Updated the Heroku deployment article

Author: xabbuh

deployment/heroku.rst

@@ -85,26 +85,16 @@ below:
 ~~~~~~~~~~~~~~~~~~~~
 
 By default, Heroku will launch an Apache web server together with PHP to serve
-applications. However, two special circumstances apply to Symfony applications:
-
-#. The document root is in the ``web/`` directory and not in the root directory
-   of the application;
-#. The Composer ``bin-dir``, where vendor binaries (and thus Heroku's own boot
-   scripts) are placed, is ``bin/`` , and not the default ``vendor/bin``.
-
-.. note::
-
-    Vendor binaries are usually installed to ``vendor/bin`` by Composer, but
-    sometimes (e.g. when running a Symfony Standard Edition project!), the
-    location will be different. If in doubt, you can always run
-    ``composer config bin-dir`` to figure out the right location.
+applications. However, a special circumstance apply to Symfony applications:
+the document root is in the ``web/`` directory and not in the root directory
+of the application.
 
 Create a new file called ``Procfile`` (without any extension) at the root
 directory of the application and add just the following content:
 
 .. code-block:: text
 
-    web: bin/heroku-php-apache2 web/
+    web: vendor/bin/heroku-php-apache2 web/
 
 .. note::
 
@@ -114,14 +104,14 @@ directory of the application and add just the following content:
 
     .. code-block:: text
 
-        web: bin/heroku-php-nginx -C nginx_app.conf web/
+        web: vendor/bin/heroku-php-nginx -C nginx_app.conf web/
 
 If you prefer working on the command console, execute the following commands to
 create the ``Procfile`` file and to add it to the repository:
 
 .. code-block:: terminal
 
-    $ echo "web: bin/heroku-php-apache2 web/" > Procfile
+    $ echo "web: vendor/bin/heroku-php-apache2 web/" > Procfile
     $ git add .
     $ git commit -m "Procfile for Apache and PHP"
     [master 35075db] Procfile for Apache and PHP