refactor: typos fixed in UploadWindow.js

bin/backend/controls/UploadWindow.js

@@ -57,7 +57,7 @@ define('package/quiqqer/ckeditor4/bin/backend/controls/UploadWindow', [
             var Content = this.getContent();
 
             this.$Upload = new UploadForm({
-                multible    : false,
+                multiple    : false,
                 sendbutton  : false,
                 cancelbutton: false,
                 events      : {