Sort columns in the ‘Pages’ dashboard overview

I just realized that there was a typo in the code. The meta-tag should read ‘prod_hierar’ instead of ‘prod-hier’. This fixes the issue. (Too much experimenting, I guess ..).

Function ‘wms_hierarchy_col_orderby()‘ thus becomes:

function wms_hierarchy_col_orderby( $vars ) {
    if ( isset( $vars['orderby'] ) && 'prod_hierar' == $vars['orderby'] ) {
        $vars = array_merge( $vars, array(
            'meta_key' => 'prod_hierar',
            'orderby' => 'meta_value'
        ) );
    }
    return $vars;
}