fix async 语法错误

src/features/OnlineExam/Examing/ExamingHome.vue

@@ -181,7 +181,7 @@ export default {
       5 * 1000 // 10秒检查是否有更改需要提交答案
     );
   },
-  mounted() {
+  async mounted() {
     // iview bug: https://github.com/iview/iview/issues/4061
     // document.body.style = "";