bug symfony#5266 Update heroku.rst (nickbyfleet)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes symfony#5266).

Discussion
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Update heroku.rst

Fixing a typo... it's buildpacks, not buildpack when using the heroku toolchain.

Commits
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b1fcee9 Update heroku.rst

cookbook/deployment/heroku.rst

@@ -301,7 +301,7 @@ This is also very useful to build assets on the production system, e.g. with Ass
 
     .. code-block:: bash
 
-        $ heroku buildpack:set https://github.com/ddollar/heroku-buildpack-multi.git
+        $ heroku buildpacks:set https://github.com/ddollar/heroku-buildpack-multi.git
 
     Next, add a ``.buildpacks`` file to your project, listing the buildpacks you need: