Fix link with Deltacomb mesh file

I don't know why this one didn't change along. Thanks to the automated tests to catch that for me.

resources/definitions/deltacomb.def.json

@@ -10,7 +10,7 @@
         "category":                        "Other",
         "file_formats":                    "text/x-gcode",
         "icon":                            "icon_ultimaker2",
-        "platform":                        "deltacomb.stl",
+        "platform":                        "deltacomb.3mf",
         "has_machine_quality":             true,
         "has_materials":                   true,
         "has_variants":                    true,