[css-mixins-1] obvious typo

Author: tabatkins

css-mixins-1/Overview.bs

@@ -586,7 +586,7 @@ with its [=function parameters=] overriding "inherited" custom properties of the
 		3. Add a [=custom property=] to |argument rule|
 			with a name of the parameter's name,
 			and a value of ''first-valid(|arg value|, |default value|)''.
-	9. [=Resolve function styles=] using |custom function|, |argument styles|, |registrations|, and |calling context|.
+	9. [=Resolve function styles=] using |custom function|, |argument rule|, |registrations|, and |calling context|.
 		Let |argument styles| be the result.
 	10. Let |body rule| be the [=function body=] of |custom function|,
 		as a [=style rule=].