Improve specification of conductor_bsdf defaults (#2272)

This changelist improves the specification of the default values for the `ior` and `extinction` inputs of `conductor_bsdf`, aligning them with the data definition of this node.

See https://github.com/AcademySoftwareFoundation/MaterialX/blob/main/libraries/pbrlib/pbrlib_defs.mtlx#L82 for reference.

Author: jstone-lucasfilm

documents/Specification/MaterialX.PBRSpec.md

@@ -193,8 +193,8 @@ The PBS nodes also make use of the following standard MaterialX types:
 
 * **`conductor_bsdf`**: Constructs a reflection BSDF based on a microfacet reflectance model[^Burley2012]. Uses a Fresnel curve with complex refraction index for conductors/metals. If an artistic parametrization[^Gulbrandsen2014] is needed the [<artistic_ior>](#node-artistic-ior) utility node can be connected to handle this.
     * `weight` (float): Weight for this BSDF’s contribution, range [0.0, 1.0]. Defaults to 1.0.
-    * `ior `(color3): Index of refraction. Defaults to (0.18, 0.42, 1.37) (approximate IOR for gold).
-    * `extinction` (color3): Extinction coefficient. Defaults to (3.42, 2.35, 1.77) (approximate extinction coefficients for gold).
+    * `ior `(color3): Index of refraction. Defaults to (0.183, 0.421, 1.373) (approximate IOR for gold).
+    * `extinction` (color3): Extinction coefficient. Defaults to (3.424, 2.346, 1.770) (approximate extinction coefficients for gold).
     * `roughness` (vector2): Surface roughness. Defaults to (0.05, 0.05).
     * `thinfilm_thickness` (float): The thickness of an iridescent thin film layer[^Belcour2017] applied over the base bsdf, expressed in nanometers. Defaults to 0.0, for no thin film.
     * `thinfilm_ior` (float): The index of refraction of the thin film layer. Defaults to 1.5.