Like this?
// About us
ob_start();
function my_tabout_shortcode() {
$tabout = get_field('tabout', 'option');?>
<div class="tabout" id="tabout"><span> <?php echo $tabout; ?> </span></div>
<?php
}
add_shortcode( 'my_tabout', 'my_tabout_shortcode' );
return ob_get_clean();