Update index.njk (#279)

Author: kt3k

source/layout/index.njk

@@ -11,8 +11,8 @@
 
 <ul>
   <li><a href="https://nodejs.org/api/">Node.js 最新</a>
+  <li><a href="https://nodejs.org/dist/latest-v12.x/docs/api/">Node.js v12.x</a>
   <li><a href="https://nodejs.org/dist/latest-v10.x/docs/api/">Node.js v10.x</a>
-  <li><a href="https://nodejs.org/dist/latest-v8.x/docs/api/">Node.js v8.x</a>
 </ul>
 
 <h2>Slack</h2>