How to add post title below the featured image in the plugin code?

Personally I would not modify the plugin, as you’ll just lose all your changes on each and every update.

But if you insist, just change line 4 to

$string .= '<li><a href="'. get_permalink() .'">'. get_the_post_thumbnail() .'</a><h6>'. get_the_title() .'</h6></li>';

Then you can target those h6s with CSS