htmlcssguide: Update TODO format (#884)

This update changes the TODO formatting and examples to more closely align with Google’s internal TODO style.

htmlcssguide.html

@@ -143,16 +143,14 @@ depends on the project’s complexity.)</p>
 <p>Highlight todos by using the keyword <code>TODO</code> only, not other common formats like
 <code>@@</code>.</p>
 
-<p>Append a contact (username or mailing list) in parentheses as with the format
-<code>TODO(contact)</code>.</p>
+<p>Append action items after a colon as in <code>TODO: action
+item</code>.</p>
 
-<p>Append action items after a colon as in <code>TODO: action item</code>.</p>
-
-<pre><code class="language-django external good">{# TODO(john.doe): revisit centering #}
+<pre><code class="language-django external good">{# TODO: Revisit centering. #}
 &lt;center&gt;Test&lt;/center&gt;
 </code></pre>
 
-<pre><code class="language-html good">&lt;!-- TODO: remove optional tags --&gt;
+<pre><code class="language-html good">&lt;!-- TODO: Remove optional tags. --&gt;
 &lt;ul&gt;
   &lt;li&gt;Apples&lt;/li&gt;
   &lt;li&gt;Oranges&lt;/li&gt;