Correct the Subtitle tag. Local single still not working over the theme though.

layouts/_default/single.html

@@ -13,8 +13,8 @@
 
         <article>
             <h2 class="post-title"><a href="{{ .Permalink }}">{{ .Title | markdownify }}</a></h2>
-            {{ if .Params.Subtitle }}
-            <strong>{{ .Params.Subtitle }}</strong>
+            {{ if .Params.subtitle }}
+            <strong>{{ .Params.subtitle | markdownify }}</strong>
             {{ end }}
 
             {{ if .Params.Cover }}