updates hgroup example (#2228)

per recent changes to `hgroup` element, reflected in the updated content of the [`hgroup`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/hgroup) MDN doc, the try it example needed to be updated as well.

Author: scottaohara

live-examples/html-examples/content-sectioning/hgroup.html

@@ -1,5 +1,5 @@
 <hgroup>
     <h1>Calculus I</h1>
-    <h2>Fundamentals</h2>
+    <p>Fundamentals</p>
 </hgroup>
 <p>This course will start with a brief introduction about the limit of a function. Then we will describe how the idea of derivative emerges in the Physics and Geometry fields. After that, we will explain that the key to master calculus is &hellip;</p>