Add tabindex to popover buttons (web-platform-tests#39770) (web-platf…

…orm-tests#40689)

I forgot one change when doing web-platform-tests#39770.

html/semantics/popovers/popover-focus-2.html

@@ -96,7 +96,7 @@
   <button id=circular2 popovertarget=popover4 popovertargetaction=show tabindex="0"></button>
   <button id=circular3 popovertarget=popover4 tabindex="0"></button>
 </div>
-<button id=circular4>after</button>
+<button id=circular4 tabindex="0">after</button>
 <script>
 promise_test(async t => {
   circular0.focus();