Update custom element examples to use new define function

Re-fixes #431. Closes #446. Also updates acknowledgments to be easier to
maintain, and adds @rniwa who was missing (in addition to @rianby64 who
spotted this and proposed #446).

spec/custom/index.html

@@ -176,7 +176,7 @@ <h4 id="custom-elements-custom-tag-example">Creating a custom tag</h4>
 <p>We then need to use this class to define the element:</p>
 
 <pre class="highlight">
-document.defineElement("flag-icon", FlagIcon);</pre>
+customElements.define("flag-icon", FlagIcon);</pre>
 
 <p>At this point, our above code will work! The parser, whenever it sees the <code>flag-icon</code> tag, will construct a new instance of our <code>FlagIcon</code> class, and tell our code about its new <code>country</code> attribute, which we then use to set the element's internal state and update its rendering (when appropriate).</p>
 
@@ -219,7 +219,7 @@ <h4 id="custom-elements-type-extension-example">Creating a type extension</h4>
 <p>When registering our custom element, we have to also specify the <code>extends</code> option:</p>
 
 <pre class="highlight">
-document.defineElement("plastic-button", PlasticButton, { extends: "button" });</pre>
+customElements.define("plastic-button", PlasticButton, { extends: "button" });</pre>
 
 <p>In general, the name of the element being extended cannot be determined simply by looking at what element interface it extends, as many elements share the same interface (such as <code>q</code> and <code>blockquote</code> both sharing <code>HTMLQuoteElement</code>).</p>
 
@@ -1139,7 +1139,29 @@ <h2>Acknowledgments</h2>
     <p>The &lt;flag-icon> example was inspired by <a itemprop="work" href="https://customelements.io/stevenrskelton/flag-icon/">a custom element</a> by <a itemprop="https://creativecommons.org/ns#attributionURL" href="http://stevenskelton.ca/">Steven Skelton</a>. (<a itemprop="license" href="https://opensource.org/licenses/MIT">MIT</a>)</p>
 </div>
 
-<p>The editor would also like to thank <span class="vcard">Alex Komoroske</span>, <span class="vcard">Anne van Kesteren</span>, <span class="vcard">Boris Zbarsky</span>, <span class="vcard">Daniel Buchner</span>, <span class="vcard">Edward O'Connor</span>, <span class="vcard">Erik Arvidsson</span>, <span class="vcard">Elliott Sprehn</span>, <span class="vcard">Hayato Ito</span>, <span class="vcard">Jan Miksovsky</span>, <span class="vcard">Jonas Sicking</span>, <span class="vcard">Olli Pettay</span>, <span class="vcard">Rafael Weinstein</span>, <span class="vcard">Scott Miles</span>, <span class="vcard">Simon Pieters</span>, <span class="vcard">Steve Orvell</span>, <span class="vcard">Tab Atkins</span>, and <span class="vcard">William Chen</span> for their comments and contributions to this specification.</p>
+<p>The editor would also like to thank
+Alex Komoroske,
+Andres Rios,
+Anne van Kesteren,
+Boris Zbarsky,
+Daniel Buchner,
+Edward O'Connor,
+Erik Arvidsson,
+Elliott Sprehn,
+Hayato Ito,
+Jan Miksovsky,
+Jonas Sicking,
+Olli Pettay,
+Rafael Weinstein,
+Ryosuke Niwa,
+Scott Miles,
+Simon Pieters,
+Steve Orvell,
+Tab Atkins,
+and
+William Chen
+
+for their comments and contributions to this specification.</p>
 
 <p>This list is too short. There's a lot of work left to do. Please contribute by reviewing and filing bugs&mdash;and don't forget to ask the editor to add your name into this section.</p>