Fix display of unlinking button

It was not being displayed with Qt 5.15 due to the following error:
MaterialsView.qml:34:5: Unable to assign QStringList to QString
This is correct. And it should not just concatenate all of these material names; it should add a comma between them for human-readable display in the text label above the button.

Fixes #10235.

resources/qml/Preferences/Materials/MaterialsView.qml

@@ -39,14 +39,14 @@ TabView
         }
         if (!base.containerId || !base.editingEnabled || !base.currentMaterialNode)
         {
-            return ""
+            return "";
         }
         var linkedMaterials = Cura.ContainerManager.getLinkedMaterials(base.currentMaterialNode, true);
         if (linkedMaterials.length == 0)
         {
-            return ""
+            return "";
         }
-        return linkedMaterials;
+        return linkedMaterials.join(", ");
     }
 
     function getApproximateDiameter(diameter)