docs: standardize on node-js-app as the app name

We’ll probably want to update this to whatever the hip language of the day is, but for now, having the same reference everywhere works.

docs/deployment/methods/dockerfiles.md

@@ -34,7 +34,7 @@ ENTRYPOINT ["node"]
 You can do:
 
 ```shell
-dokku config:set APP DOKKU_DOCKERFILE_START_CMD="--harmony server.js"
+dokku config:set node-js-app DOKKU_DOCKERFILE_START_CMD="--harmony server.js"
 ```
 
 To tell docker what to run.