Update specification link

Co-authored-by: Gary Hsu <bghgary@users.noreply.github.com>

loaders/src/glTF/2.0/Extensions/KHR_materials_variants.ts

@@ -29,7 +29,8 @@ interface VariantMapping {
 }
 
 /**
- * [Specification](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_materials_variants)
+ * [Proposed Specification](https://github.com/KhronosGroup/glTF/pull/1681)
+ * !!! Experimental Extension Subject to Changes !!!
  */
 export class KHR_materials_variants implements IGLTFLoaderExtension {
   /**
@@ -128,4 +129,4 @@ export class KHR_materials_variants implements IGLTFLoaderExtension {
   dispose = () => {};
 }
 
-GLTFLoader.RegisterExtension(NAME, (loader) => new KHR_materials_variants(loader));
+GLTFLoader.RegisterExtension(NAME, (loader) => new KHR_materials_variants(loader));