Update glTF extension spec links

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

@@ -11,7 +11,7 @@ import { IKHRMaterialsSheen } from 'babylonjs-gltf2interface';
 const NAME = "KHR_materials_sheen";
 
 /**
- * [Proposed Specification](https://github.com/KhronosGroup/glTF/pull/1688)
+ * [Specification](https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_materials_sheen/README.md)
  * [Playground Sample](https://www.babylonjs-playground.com/frame.html#BNIZX6#4)
  * !!! Experimental Extension Subject to Changes !!!
  */

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

@@ -250,8 +250,7 @@ class TransmissionHelper {
 const NAME = "KHR_materials_transmission";
 
 /**
- * [Proposed Specification](https://github.com/KhronosGroup/glTF/pull/1698)
- * !!! Experimental Extension Subject to Changes !!!
+ * [Specification](https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_materials_transmission/README.md)
  */
 export class KHR_materials_transmission implements IGLTFLoaderExtension {
     /**

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

@@ -21,8 +21,7 @@ interface IExtensionMetadata {
 }
 
 /**
- * [Proposed Specification](https://github.com/KhronosGroup/glTF/pull/1681)
- * !!! Experimental Extension Subject to Changes !!!
+ * [Specification](https://github.com/KhronosGroup/glTF/blob/master/extensions/2.0/Khronos/KHR_materials_variants/README.md)
  */
 export class KHR_materials_variants implements IGLTFLoaderExtension {
     /**