Update field_sortable.php

The label should stay the same so people know what that field is for. Right now it just displays the input from the textfield (redundant).

Ideally might be best to have an array for each option

'key' => array( 'input', 'label' ) - or something of the sort. I'll think about it.

In my case I use this for social options, so under each field the label should read "Twitter, Facebook, Dribble...etc" and the field value should have a sample URL.
Merge pull request #1365 from wpexplorer/patch-1

Update field_sortable.php