[css-mixins-1] reword

Author: tabatkins

css-mixins-1/Overview.bs

@@ -1119,7 +1119,7 @@ similar to an unregistered [=custom property=].
 	</xmp>
 
 	In the above code, the [=mixin parameter=]
-	is set to a [=custom property=] on the parent element,
+	is used to set a [=custom property=] on the parent element,
 	where it's forced to resolve as a length
 	due to being passed through the [=custom function=].
 	All the styles then use that [=custom property=]