update webpack link

Author: dingyiming

themes/vue/layout/partials/main_menu.ejs

@@ -4,6 +4,6 @@
 <li><a href="/vue-router/" class="nav-link<%- page.path.match(/vue-router/) ? ' current' : '' %>">vue-router</a></li>
 <li><a href="/vuex/" class="nav-link<%- page.path.match(/vuex/) ? ' current' : '' %>">vuex</a></li>
 <li><a href="/examples/" class="nav-link<%- page.path.match(/examples/) ? ' current' : '' %>">示例</a></li>
-<li><a href="http://webpack.vuefe.cn" target="_blank">webpack</a></li>
+<li><a href="https://webpack2.leanapp.cn" target="_blank">webpack</a></li>
 <li><a href="/more/" class="nav-link<%- page.path.match(/more/) ? ' current' : '' %>">更多</a></li>
 <%- partial('partials/community_dropdown') %>