Change Yoast SEO NoIndex Meta Robots Tag

Yoast SEO was adding noindex to the robots meta tag on all custom post types archives. The following code changes it to index.

/* change noindex tag for cpt archives */
add_filter( 'wpseo_robots', function( $string ) {
	if ( is_post_type_archive() )
		return 'index, follow';
	else
		return $string;
});