Docs: Update a link to the Custom Elements spec in unsupported_valid_tag_names() unit test docblock.

This changeset replaces a link to the outdated W3C specs on Custom Elements with a link to the corresponding WhatWG specification.

Previously committed in [53205], this appears to be accidentally reverted in [53562].

Follow-up to [53204], [53205], [53562].

Props audrasjb.
See #56792.

git-svn-id: https://develop.svn.wordpress.org/trunk@54727 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/formatting/balanceTags.php

@@ -80,7 +80,7 @@ class Tests_Formatting_BalanceTags extends WP_UnitTestCase {
 	/**
 	 * These are valid custom elements but we don't support them yet.
 	 *
-	 * @see https://w3c.github.io/webcomponents/spec/custom/#valid-custom-element-name
+	 * @see https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name
 	 */
 	public function unsupported_valid_tag_names() {
 		return array(