How do I remove a particular tag name from the tag cloud ?

I changed wp-includes/default-widgets.php on line 1038 from

wp_tag_cloud( apply_filters('widget_tag_cloud_args', array('taxonomy' => $current_taxonomy) ) );

to

wp_tag_cloud( apply_filters('widget_tag_cloud_args', array('taxonomy' => $current_taxonomy, 'exclude' => "featured") ) );

But no effect.