Fix typo in upgrading docs
Fix typo in upgrading docs

it's "its" not "it's".
claytonbrown/dokku,alexquick/dokku,jimjag/dokku,dokku/dokku,progrium/dokku,dokku/dokku,claytonbrown/dokku,progrium/dokku,progrium/dokku,jimjag/dokku,jimjag/dokku,jimjag/dokku,claytonbrown/dokku,claytonbrown/dokku,claytonbrown/dokku,alexquick/dokku,dokku/dokku,progrium/dokku,progrium/dokku,alexquick/dokku,progrium/dokku,alexquick/dokku,progrium/dokku,claytonbrown/dokku,progrium/dokku,dokku/dokku,jimjag/dokku,alexquick/dokku,alexquick/dokku,jimjag/dokku,dokku/dokku,dokku/dokku,jimjag/dokku,dokku/dokku,jimjag/dokku,alexquick/dokku,alexquick/dokku,dokku/dokku,claytonbrown/dokku,claytonbrown/dokku