docs: remove digitalocean instance size reference.
docs: remove digitalocean instance size reference.

[ci skip]
dokku/dokku,alexquick/dokku,alexquick/dokku,progrium/dokku,claytonbrown/dokku,alexquick/dokku,jimjag/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,alexquick/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,alexquick/dokku,claytonbrown/dokku,dokku/dokku,alexquick/dokku,claytonbrown/dokku,dokku/dokku,jimjag/dokku,progrium/dokku,dokku/dokku,jimjag/dokku,alexquick/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,claytonbrown/dokku,dokku/dokku,progrium/dokku,progrium/dokku,alexquick/dokku,jimjag/dokku,jimjag/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,jimjag/dokku