Update webxr_xr_sculpt.html

Add missing namespace.

examples/webxr_xr_sculpt.html

@@ -133,7 +133,7 @@
 					path + 'pz' + format, path + 'nz' + format
 				];
 
-				let reflectionCube = new CubeTextureLoader().load( urls );
+				let reflectionCube = new THREE.CubeTextureLoader().load( urls );
 				*/
 
 				const material = new THREE.MeshStandardMaterial( {