Update DirectionalLightHelper.html (#27122)

Add scene.add(light) to fix example code

docs/api/en/helpers/DirectionalLightHelper.html

@@ -22,6 +22,7 @@
 		<code>
 		const light = new THREE.DirectionalLight( 0xFFFFFF );
 		const helper = new THREE.DirectionalLightHelper( light, 5 );
+		scene.add( light );
 		scene.add( helper );
 		</code>