Fix improper grammar in Dutch i18n file

Grammar is incorrect when maximum is 1, 'kunnen' should be 'kan' in that case.

src/js/select2/i18n/nl.js

@@ -23,7 +23,8 @@ define(function () {
       return 'Meer resultaten laden…';
     },
     maximumSelected: function (args) {
-      var message = 'Er kunnen maar ' + args.maximum + ' item';
+      var verb = args.maximum == 1 ? 'kan' : 'kunnen';
+      var message = 'Er ' + verb + ' maar ' + args.maximum + ' item';
 
       if (args.maximum != 1) {
         message += 's';