Little correction to dataprovider.

There is a little correction to the method dataprovider, the parameters for the callback function are index, value.  Probably the problem is that in the forEach method of an array this two parameters are inverted.

js/bootstrap-multiselect.js

@@ -894,7 +894,7 @@
             var optionDOM = "";
             var groupCounter = 0;
 
-            $.each(dataprovider, function (option) {
+            $.each(dataprovider, function (index, option) {
                 if ($.isArray(option.children)) {
                     groupCounter++;
                     optionDOM += '<optgroup label="' + (option.title || 'Group ' + groupCounter) + '">';