Added addons:downgrade alias

packages/cli/src/commands/addons/upgrade.ts

@@ -8,6 +8,7 @@ import {HTTP} from 'http-call'
 import {HerokuAPIError} from '@heroku-cli/command/lib/api-client'
 
 export default class Upgrade extends Command {
+  static aliases = ['addons:downgrade'];
   static topic = 'addons'
   static description = 'change add-on plan'
   static help = 'See available plans with `heroku addons:plans SERVICE`.\n\nNote that `heroku addons:upgrade` and `heroku addons:downgrade` are the same.\nEither one can be used to change an add-on plan up or down.\n\nhttps://devcenter.heroku.com/articles/managing-add-ons'