Call letter subjects ‘main heading’

This is a term that one of our research participants used to describe
the big bold text that starts each letter. I think it’s quite a nice
plain english term for it.

Also changes the formatting guidance to use the word heading instead of
title, for consistency.

app/templates/partials/templates/guidance-formatting-letters.html

@@ -1,10 +1,10 @@
 <h2 class="heading-medium">Formatting</h2>
 <p>
-  To put a title in your template, use a hash:
+  To put a heading in your template, use a hash:
 </p>
 <div class="panel panel-border-wide">
   <p>
-    # This is a title
+    # This is a heading
   </p>
 </div>
 <p>