fix doc version 8.1.0 link error (#114)

* fix doc version 8.1.0 link error

* fix chinese version 8.1.0 link error
diff --git a/docs/.vuepress/config.js b/docs/.vuepress/config.js
index 2c3cd28..ef9d756 100755
--- a/docs/.vuepress/config.js
+++ b/docs/.vuepress/config.js
@@ -27,7 +27,7 @@
         nav: [
           { text: "Home", link: "/" },
           { text: "Docs",  items: [
-            { text: '8.1.0', link: "https://github.com/apache/skywalking/tree/v8.0.1/docs"},
+            { text: '8.1.0', link: "https://github.com/apache/skywalking/tree/v8.1.0/docs"},
             { text: '8.0.1', link: "https://github.com/apache/skywalking/tree/v8.0.1/docs"},
             { text: '7.0.0', link: "https://github.com/apache/skywalking/tree/v7.0.0/docs"},
             { text: 'Nginx LUA 0.1.0', link: "https://github.com/apache/skywalking-nginx-lua/tree/v0.1.0"},
@@ -78,7 +78,7 @@
           { text: "主页", link: "/zh/" },
           { text: "文档",
             items: [
-              { text: '8.1.0', link: "https://github.com/apache/skywalking/tree/v8.0.1/docs"},
+              { text: '8.1.0', link: "https://github.com/apache/skywalking/tree/v8.1.0/docs"},
               { text: '8.0.1', link: "https://github.com/apache/skywalking/tree/v8.0.1/docs"},
               { text: '7.0.0', link: "https://github.com/apache/skywalking/tree/v7.0.0/docs"},
               { text: 'Nginx LUA 0.1.0', link: "https://github.com/apache/skywalking-nginx-lua/tree/v0.1.0"},