Change Heroku button repo.

Author: knadh

README.md

@@ -47,7 +47,7 @@ See [installation docs](https://listmonk.app/docs/installation).
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 ### Heroku 
-[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/knadh/listmonk-heroku)
+[![Deploy](https://www.herokucdn.com/deploy/button.svg)](https://heroku.com/deploy?template=https://github.com/knadh/listmonk-heroku-deploy)
 
 
 ## Developers