fix sample code

Author: robertsulej

examples/1_basic_plot_making/4_properties_of_materials.ipynb

@@ -225,10 +225,10 @@
    "outputs": [],
    "source": [
     "m_clear_glass[\"VarFloat3\"][\"refraction_index\"] = [1.38, 1.4, 1.43]\n",
-    "optix.update_material(\"glass\", m_clear_glass, refresh=True)\n",
+    "optix.update_material(\"glass\", m_clear_glass)\n",
     "\n",
     "m_air[\"VarFloat3\"][\"refraction_index\"] = [1.38, 1.4, 1.43] # same as in glass, so particles nested in the sphere are not visible\n",
-    "optix.update_material(\"air\", m_clear_glass, refresh=True)"
+    "optix.update_material(\"air\", m_air, refresh=True)"
    ]
   },
   {
@@ -245,7 +245,7 @@
    "outputs": [],
    "source": [
     "m_air[\"VarFloat3\"][\"refraction_index\"] = [1, 1, 1]\n",
-    "optix.update_material(\"air\", m_clear_glass, refresh=True)"
+    "optix.update_material(\"air\", m_air, refresh=True)"
    ]
   },
   {
@@ -385,7 +385,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.7.4"
+   "version": "3.6.9"
   }
  },
  "nbformat": 4,