Updating to make post title it's H1 on single page

Right now there wasn't an H1 at all, and the title was written out as a H2. Ideally we'd have the H1 be the main subject of the page (the post title seems appropriate for that), the first heading and there would be only the one H1.

layouts/posts/single.html

@@ -15,7 +15,7 @@
         </div>
 
         <article>
-            <h2 class="post-title"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h2>
+            <h1 class="post-title"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h1>
 
             {{- if .Params.toc }}
                 <hr />