[Plantuml] Bad hierarchy for Meeting.GroupProposalStatus

Hello Sir, 

I'm learning DDD and using PlantUML, but I'm having a problem with the [Conceptual_Model.puml](https://github.com/kgrzybek/modular-monolith-with-ddd/blob/master/docs/PlantUML/Conceptual_Model.puml), i can't visualize that because of a Hierarchy problem