Disappointed not to have received assistance but i managed to do it on my own like that (for users and searchengine) :
I added tag
<h3></h3>
to $heading includes/shortcodes.php
<?php
$heading = sanitize_text_field($heading);
$heading = apply_filters( 'mtphr_toggle_heading', '<a href="#"><span class="mtphr-toggle-button mtphr-toggle-button-condensed">+</span><span class="mtphr-toggle-button mtphr-toggle-button-expanded">–</span><h3>'.$heading.'</h3></a>', $heading, $id );
$content = apply_filters( 'mtphr_toggle_content', apply_filters('the_content', mtphr_shortcodes_parse_shortcode_content($content)), $id );
?>
And in personnal CSS :
.mtphr-toggle-button {
margin-top: 12px;
}
.mtphr-toggle h3 {
margin-left: 28px;
}