How to add a data attribute to a WordPress menu item

Specifically editing the code you provided in the original question:

add_filter( 'nav_menu_link_attributes', 'wpse121123_contact_menu_atts', 10, 3 );
function wpse121123_contact_menu_atts( $atts, $item, $args )
{
  // The ID of the target menu item
  $menu_target = 123;

  // inspect $item
  if ($item->ID == $menu_target) {
    $atts['data-toggle'] = 'modal';
  }
  return $atts;
}