Fix focusable area example to use non-null tabindex

Closes #5262.

source

@@ -74113,8 +74113,8 @@ END:VCARD</pre>
      Elements that meet all the following criteria:
 
      <ul class="brief">
-      <li>the element's <span>tabindex value</span> is a non-negative integer, or the element is
-      determined by the user agent to be focusable;</li>
+      <li>the element's <span>tabindex value</span> is non-null, or the element is determined by the
+      user agent to be focusable;</li>
 
       <li>the element is either not a <span>shadow host</span>, or has a <span
       data-x="concept-element-shadow-root">shadow root</span> whose <span>delegates focus</span> is