Fix blog sidenav.

This got broken in https://github.com/prometheus/docs/pull/365

layouts/blog_sidebar.html

 <div class="col-md-3 side-nav-col">
   <ul class="nav navbar-nav side-nav">
-    <li>
+    <li class="active">
       <span class="nav-header">Blog posts</span>
-      <ul class="nav">
+      <ul class="nav active">
       <% sorted_articles.each do |post| %>
         <li><%= link_to post[:title], post.path %></li>
       <% end %>

static/docs.css

@@ -108,7 +108,7 @@ a.sc-logo img {
   font-size: 16px;
 }
 
-.side-nav .nav-header a, .side-nav .nav-header span {
+.side-nav .nav-header a, .side-nav .nav-header {
   color: #e6522c;
   text-transform: uppercase;
   text-decoration: none;