Update Readme.md

Readme.md

@@ -35,7 +35,7 @@ Example usage:
 
 You can also add it to upcoming builds of an existing application:
 
-    $ heroku config BUILDPACK_URL=git@github.com:heroku/heroku-buildpack-python.git
+    $ heroku config:add BUILDPACK_URL=git@github.com:heroku/heroku-buildpack-python.git
 
 The buildpack will detect your app as Python if it has the file `requirements.txt` in the root. It will detect your app as Python/Django if there is an additional `settings.py` in a project subdirectory.