Update matching-characters.md (act-rules#2144)

* Update matching-characters.md

* Update pages/glossary/matching-characters.md

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>

---------

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>

Author: kengdoj

pages/glossary/matching-characters.md

@@ -5,4 +5,6 @@ unambiguous: true
 objective: true
 ---
 
-A sequence of characters is considered to _match_ another if, after removing leading and trailing [space characters](https://html.spec.whatwg.org/#white_space) and replacing remaining occurrences of one or more space characters with a single space, the two sequences of characters are equal character-by-character, ignoring any differences in letter casing.
+A sequence of characters is considered to _match_ another if, after removing leading and trailing [whitespace characters][] and replacing remaining occurrences of one or more whitespace characters with a single space, the two sequences of characters are equal character-by-character, ignoring any differences in letter casing.
+
+[whitespace characters]: #whitespace 'Definition of Whitespace'