fix(dgeni): interface property heading tags don't respect child mode (#3538)

Author: griest024

tools/dgeni/src/templates/api/interface.template.html

@@ -10,7 +10,7 @@
 <span class="selectors__name"><code>{$ doc.decorators[0].argumentInfo[0].selector $}</code></span>
 {% endif %}
 
-<h2>Properties</h2>
+{$'<h4>' if child else '<h2>'$}Properties{$'</h4>' if child else '</h2>'$}
 <table>
   <thead>
     <tr>