Update CubeCamera.html

The `.setVisible` method is not in the Mesh documentation, or in the Object3D documentation. Changed this according to here: mrdoob#251 (comment)

docs/api/en/cameras/CubeCamera.html

@@ -29,12 +29,12 @@ <h2>Examples</h2>
 		scene.add( car );
 
 		// Update the render target cube
-		car.setVisible( false );
+		car.visible = false;
 		cubeCamera.position.copy( car.position );
 		cubeCamera.update( renderer, scene );
 
 		// Render the scene
-		car.setVisible( true );
+		car.visible = true;
 		renderer.render( scene, camera );
 		</code>