Merge pull request #240 from Authing/doc-update-v4-link

doc: update 4.x link

Author: zhaoyiming0803

docs/.vuepress/theme/components/Navbar.vue

@@ -84,7 +84,7 @@ export default {
         text: version
       }, {
         text: '4.x',
-        link: 'https://docs.authing.cn/v2/reference/guard/v2/react.html'
+        link: 'https://docs.authing.cn/v2/reference/guard/v2/guard-v4.html'
       }]
     };
   },