Add xlink namespace to URL validation error type XML (#5481)

Author: leonardolara

reference/uri/uri.whatwg.urlvalidationerrortype.xml

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<reference xml:id="enum.uri-whatwg-urlvalidationerrortype" role="enum" xmlns="http://docbook.org/ns/docbook">
+<reference xml:id="enum.uri-whatwg-urlvalidationerrortype" role="enum" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://docbook.org/ns/docbook">
  <title>The Uri\WhatWg\UrlValidationErrorType enum</title>
  <titleabbrev>Uri\WhatWg\UrlValidationErrorType</titleabbrev>