docs: cleanup headings in nginx plugin
docs: cleanup headings in nginx plugin

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