Fix h2 tag so heading displays properly

app/templates/views/documentation.html

@@ -23,7 +23,7 @@
     {% endfor %}
   </ul>
     <p>A developer should be able to set up the API client and start sending test messages in around 30 minutes. A full integration can take a few days, depending on the other systems you’re using.</p>
-  <h2>Integrating directly with the API</h2>
+  <h2 class="heading-medium">Integrating directly with the API</h2>
     <p>We recommend using the client libraries rather than integrating directly with the API.</p>
     <p>There’s no documentation for using the API in this way. You’ll still need to read the client documentation to understand:</p>
   <ul class="list list-bullet">