Fix typo

Author: mirisuzanne

css-cascade-5/Overview.bs

@@ -1099,7 +1099,7 @@ Layering Styles: the ''@layer'' rule</h4>
 	that either matches an existing layer name, or defines a new named layer.
 	If no identifier is given, a new unnamed layer is created.
 	The '<<url>>' import or '<<stylesheet>>' block
-	provides a arbitrary CSS rules that will be appended to the layer.
+	provides arbitrary CSS rules that will be appended to the layer.
 
 	The ''@layer'' rule can also be used to define new layers
 	without attaching any style rules, by providing only an identifier: