[docs] Mention KHR_materials_pbrSpecularGlossiness in GLTF2Loader docs

docs/examples/loaders/GLTF2Loader.html

@@ -40,6 +40,11 @@
 					KHR_materials_common
 				</a>
 			</li>
+			<li>
+				<a target="_blank" href="https://github.com/KhronosGroup/glTF/tree/2.0/extensions/Khronos/KHR_materials_pbrSpecularGlossiness">
+					KHR_materials_pbrSpecularGlossiness
+				</a>
+			</li>
 		</ul>
 
 		<h2>Example</h2>