Guide version menu should show "3.x" (vuejs#2699)

Did reference "3.x-beta"

Author: LinusBorg

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-beta</option>
+              <option value="v3">3.x</option>
               <option value="SELF" selected>2.x</option>
               <option value="v1">1.0</option>
               <option value="012">0.12</option>