[Heroku] A few more tweaks to outline the steps

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | n/a

Hey guys!

This is just a tweak to #4591 - it's just a minor organizational thing so that (hopefully) it's even more clear what 3 steps you need to take.

Thanks!
I don't like this comment. Since this is a step-by-step section, it's very clear already that you have to do (1) and (2) before doing (3). When ommitting this, you can also use the very nice `#.` list :)