Update .mtlx reference

Author: portsmouth

reference/open_pbr_surface.mtlx

@@ -69,7 +69,7 @@
            doc="Coverage weight of the thin-film. Use for materials such as multi-tone car paint or soap bubbles." />
     <input name="thin_film_thickness" type="float" value="0.5" uimin="0.0" uisoftmax="1.0" uiname="Thin Film Thickness" uifolder="Thin Film" uiadvanced="true"
            doc="The thickness of the thin-film layer on the base (in micrometers)." />
-    <input name="thin_film_ior" type="float" value="1.5" uimin="0.0" uisoftmin="1.0" uisoftmax="3.0" uiname="Thin Film Index of Refraction" uifolder="Thin Film" uiadvanced="true"
+    <input name="thin_film_ior" type="float" value="1.4" uimin="0.0" uisoftmin="1.0" uisoftmax="3.0" uiname="Thin Film Index of Refraction" uifolder="Thin Film" uiadvanced="true"
            doc="The index of refraction of the thin-film." />
     <input name="emission_luminance" type="float" value="0.0" uimin="0.0" uisoftmax="1000.0" uiname="Emission Luminance" uifolder="Emission"
            doc="The amount of emitted light, as a luminance in nits." />