Added optical properties uri to example material

Signed-off-by: ClemensLinnhoff <[email protected]>

examples/example_material.xomp

@@ -31,6 +31,7 @@
         "source": "internet: https://en.wikipedia.org/wiki/Aluminium"
     },
     "electromagneticPropertiesUri": "example_material_emp.xompt",
+    "opticalPropertiesUri": "example_material_optical.xompt",
     "brdfUris": [
       "example_material_camera_brdf.xompt",
       "example_material_lidar_brdf.xompt",