Sort WordPress Posts by Modified Date using Query Param

wp-sort.php

<?php

//put this into functions.php and add url param to homepage: sortby=updated

function custom_query_vars_filter($vars) {
  $vars[] = 'sortby';
  //$vars[] .= 'another';
  return $vars;
}
add_filter( 'query_vars', 'custom_query_vars_filter' );

function techslides_alter_query($query){
      $sortby = get_query_var( 'sortby' );
      if( $sortby=="modified" && !is_admin() && $query->is_main_query() ){
        $query->set( 'orderby', 'modified' );
      }
}
add_action('pre_get_posts','techslides_alter_query');
?>