[css-cascade-6][editorial] Replace source order with order of appearance

Author: mirisuzanne

css-cascade-6/Overview.bs

@@ -311,7 +311,7 @@ Effects of ''@scope''</h4>
 
 	* The [=cascade=] prioritizes declarations
 		with a [=scope proximity|more proximate=] [=scoping root=],
-		regardless of specificity or source order
+		regardless of specificity or order of appearance
 		by applying [=scope proximity=]
 		between the [=scoping root=] and the [=subject=]
 		of each [=scoped style rule=].