Merge pull request #55 from phyng/bug-fix

Fix spelling mistake on README.md.

Author: longbai

README.md

@@ -74,7 +74,7 @@ qiniu-js-sdk
         drop_element: 'container',        //拖曳上传区域元素的ID，拖曳文件或文件夹后可触发上传
         chunk_size: '4mb',                //分块上传时，每片的体积
         auto_start: true,                 //选择文件后自动上传，若关闭需要自己绑定事件触发上传,
-        //x_vals : {
+        //x_vars : {
         //    自定义变量，参考http://developer.qiniu.com/docs/v6/api/overview/up/response/vars.html
         //    'time' : function(up,file) {
         //        var time = (new Date()).getTime();