Update the HTML Document Type guidance

This removes the guidance around not closing void elements since this is incompatible with the Prettier formatter.

htmlcssguide.html

@@ -173,9 +173,6 @@ <h4 id="Document_Type" class="numbered">Document Type</h4>
 <a href="https://hixie.ch/advocacy/xhtml"><code>application/xhtml+xml</code></a>, lacks both browser
 and infrastructure support and offers less room for optimization than HTML.)</p>
 
-<p>Although fine with HTML, do not close void elements, i.e. write <code>&lt;br&gt;</code>, not <code>&lt;br
-/&gt;</code>.</p>
-
 <h4 id="HTML_Validity" class="numbered">HTML Validity</h4>
 
 <p>Use valid HTML where possible.</p>