[css-mixins-1][editorial] Actually exercise what the example claims its showing.

Author: tabatkins

css-mixins-1/Overview.bs

@@ -1479,7 +1479,7 @@ but ''env()'' can be used in locations that don't have an element context.
 		}
 	}
 	h1 {
-		@apply --smaller {
+		@apply --smaller(50em) {
 			margin: 1em 0;
 		}
 	}