Join buildpack sections

[SimonLab]

In https://github.com/dwyl/learn-heroku/blob/master/elixir-phoenix-app-deployment.md the section 5 could be next to the section 9 and more information about the buildpacks configuration can be added:

and

see: https://github.com/dwyl/learn-heroku/pull/18#pullrequestreview-98185047

[nelsonic]

@SimonLab agreed. But what we could consider is removing section 5 entirely because we aren't actually customising anything in the buildpack ... 🤔