Update focus-order - replace reference to HTML 4.01 spec

Replace reference to focus section of HTML 4.01 spec with reference to focus section of the HTML living standard.

understanding/20/focus-order.html

@@ -36,8 +36,7 @@ <h2>Intent of Focus Order</h2>
          via the notion of tabbing order. Dynamic HTML may modify the navigation sequence using
          scripting along with the addition of a tabindex attribute to allow focus to additional
          elements. If no scripting or tabindex attributes are used, the navigation order is
-         the order that components appear in the content stream. (See HTML 4.01 Specification,
-         section 17.11, "Giving focus to an element").
+         the order that components appear in the content stream. (See the <a href="https://html.spec.whatwg.org/multipage/interaction.html#focus">Focus section of the HTML Living Standard</a>").
       </p>
 
       <p>An example of keyboard navigation that is not the sequential navigation addressed