normalize top-level help

Fixes this inconsistency:

```console
$ heroku help | grep Cont -C 3
  orgs         # 
  pg           #  manage heroku-postgresql databases
  pgbackups    #  manage backups of heroku postgresql databases
  pipeline     #  Continuous delivery pipeline actions
  plugins      #  manage plugins to the heroku gem
  ssl          #  manage ssl certificates for an app
  stack        #  manage the stack for an app
```