Update digitalocean docs
Update digitalocean docs

They were slightly out of date. Also include an updated referral link that deeplinks to the marketplace image.

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claytonbrown/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,dokku/dokku,dokku/dokku,dokku/dokku,progrium/dokku,progrium/dokku,progrium/dokku,dokku/dokku,progrium/dokku,claytonbrown/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku