fix typo

Author: tandavas

docs/prerender.md

@@ -30,7 +30,7 @@ new PrerenderSpaPlugin(
 
 If you also wanted to prerender `/about` and `/contact`, then that array would be `[ '/', '/about', '/contact' ]`.
 
-4. Enable history mode for for `vue-router`:
+4. Enable history mode for `vue-router`:
 ```js
 const router = new VueRouter({
   mode: 'history',