update link from Magritte project to point to GsDevKitHome project page

The [Magritte project page](https://github.com/magritte-metamodel/magritte) should point at  https://github.com/GsDevKit/gsDevKitHome/tree/master/projects/magritte3 when [1.0.0](https://github.com/GsDevKit/gsDevKitHome/milestones/1.0.0) released ..
