Update HTMLPortalElement spec link in idl.

Bug: 865565
Change-Id: Ie76202a74a8d9a31884a04de88c99395800d6b5a
Reviewed-on: https://chromium-review.googlesource.com/c/1372274Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: Lucas Gadani <lfg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615672}

third_party/blink/renderer/core/html/portal/html_portal_element.idl

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// https://github.com/KenjiBaheux/portals/blob/master/explainer.md
+// https://wicg.github.io/portals/#the-portal-element
 
 [HTMLConstructor, RuntimeEnabled=Portals]
 interface HTMLPortalElement : HTMLElement {