fix: url of v3.x document (#2929)

* fix: url of v3.x document

## fix
- change url of v3.x document by `vuejs.org` instead `v3.vuejs.org`

* fix: url of v3.x document

* fix: url of v3.x document

Author: huibizhang

themes/vue/layout/page.ejs

@@ -10,7 +10,7 @@
 <div class="content <%- page.type ? page.type + ' with-sidebar' : '' %> <%- page.type === 'guide' ? page.path.replace(/.+\//, '').replace('.html', '') + '-guide' : '' %>">
   <p class="tip warning v3-warning">
     You’re browsing the documentation for v2.x and earlier.
-    For v3.x, <a href="https://v3.vuejs.org/">click here</a>.
+    For v3.x, <a href="https://vuejs.org/">click here</a>.
   </p>
 
   <% if (page.type) { %>

themes/vue/layout/partials/header.ejs

@@ -1,7 +1,7 @@
 <div>
   <div id="v3-banner">
     <span class="hidden-sm">You’re browsing the documentation for v2.x and earlier.</span>
-    <a href="https://v3.vuejs.org/">Click here</a> for v3.x documentation.
+    <a href="https://vuejs.org/">Click here</a> for v3.x documentation.
   </div>
 
   <header id="header">

themes/vue/layout/partials/sidebar.ejs

@@ -21,7 +21,7 @@
           <%- titles[type] %>
           <% if (['cookbook', 'style-guide'].indexOf(type) === -1) { %>
             <select class="version-select">
-              <option value="v3">3.x</option>
+              <option value="">3.x</option>
               <option value="SELF" selected>2.x</option>
               <option value="v1">1.0</option>
               <option value="012">0.12</option>