Update model versions wiki page to reflect Enterprise Architect 15.x functionality

[sptws001]

Enterprise Architect includes an option to update the status, version, phase, and modification date for a package branch (package and all of its sub-packages and model elements, etc.).

Prior to version 15.0, one can simply right-click on the desired package in the project browser and follow the instructions on the existing model versions page (https://github.com/ISO-TC211/UML-Best-Practices/wiki/Model-version).

Starting in Enterprise Architect 15.0, this capability has been moved to the Design ribbon and updated instructions can be found at: https://sparxsystems.com/enterprise_architect_user_guide/15.2/project_management/updatepkgstatus.html

The model versions page should be updated to reflect the latest instructions.