boxed-str: add subtitle about rust-lldb

_layouts/post.html

@@ -6,7 +6,7 @@ layout: default
   <div class="post-header">
     <h1 class="post-title">{{ page.title | markdownify }}</h1>
     {% if page.subtitle %}
-    <h2 class="post-subtitle">{{ page.subtitle }}</h1>
+    <h2 class="post-subtitle">{{ page.subtitle | markdownify }}</h1>
     {% endif %}
     <p class="post-meta">
       <span>{{ page.date | date: "%b %-d, %Y" }}</span>