use abbr tag

Author: sarahs

layouts/dev-toc.html

@@ -19,7 +19,7 @@ <h3>Versions</h3>
 </ul>
 
 {% if allVersions[currentVersion] %}
-<h2 class="mt-3 mb-3">TOC for {{ allVersions[currentVersion].versionTitle }}</h2>
+<h2 class="mt-3 mb-3"><abbr>TOC</abbr> for {{ allVersions[currentVersion].versionTitle }}</h2>
 
 <button class="btn mb-3 js-expand" type="button">Expand All</button>
 <div/>