[css-shadow-parts-1] Fix typo sets->set

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -124,7 +124,7 @@ Elements in a <a>shadow tree</a> may be exposed for styling by stylesheets outsi
 using the part and partmap attributes.
 
 Each element has a <dfn export for="element">part name list</dfn>
-which is an [=ordered sets=] of tokens.
+which is an [=ordered set=] of tokens.
 
 Each element has a <dfn export for="element">part name map</dfn>
 which is an <a>ordered map</a>,