Fixed link to parent WP in the WP table

https://www.openproject.org/work_packages/14707
What are the plans for the model_type? As long as it is only used within the context of the column-data-directive it should be safe to just use work_package as the model_type. Plus, I guess, that it would be better from an API perspective to not differentiate there. While it is only the experimental API, why not take up a good idea.