doc prev site update

src/views/questioncenter/components/paper-actions-mixin.js

@@ -15,7 +15,8 @@ export default {
       } else if (this._isOffice(url)) {
         // https://view.officeapps.live.com/op/view.aspx
         // https://view.xdocin.com/view
-        window.open('https://view.xdocin.com/view?src=' + encodeURIComponent(url))
+        const prefixUrl = 'https://view.officeapps.live.com/op/view.aspx?src='
+        window.open(prefixUrl + encodeURIComponent(url))
       } else {
         this.$message.error('格式错误，无法预览该格式文件！');
       }