adapt javascript to css on doc page

[zhlinh]

for css is:

#toc .toc-h2 {
    padding-left: 10px;
}

#toc .toc-h3 {
    padding-left: 20px;
}

adapt javascript to:

'itemClass': function(i, heading, $heading, prefix) { // custom function for item class
    return prefix + '-' + $heading[0].tagName.toLowerCase();