修改 fa fa-github 为 fab fa-github (#1692)

由于 fa-github 图标 并不在 fa class 下 而在 fab class下
导致 fa-github 图标得不到显示 所以修改为 fab

genny/newapp/web/templates/templates/index.html.tmpl

@@ -7,7 +7,7 @@
       <div class="col-md-9 col-sm-9 col-xs-10 titles">
         <h1><%= t("welcome_greeting") %></h1>
         <h2>
-          <a href="https://github.com/gobuffalo/buffalo"><i class="fa fa-github" aria-hidden="true"></i> https://github.com/gobuffalo/buffalo</a>
+          <a href="https://github.com/gobuffalo/buffalo"><i class="fab fa-github" aria-hidden="true"></i> https://github.com/gobuffalo/buffalo</a>
         </h2>
         <h2 class="documentation">
           <a href="http://gobuffalo.io/"><i class="fa fa-book" aria-hidden="true"></i> Documentation</a>