Provide more fields with the postresql url configuration for Rails

[sc-david-voisin]

On this page:
https://doc.scalingo.com/languages/ruby/rails/start#postgresql

We provide this line as example, which is correct:
url: <%= ENV["DATABASE_URL"]&.gsub(/postgres:/, 'postgis:') %>

But after a remak from a customer, it would more comprehensive with a sample of the file and only one line, such as