[css-cascade-5] Fix more stray example errors. :/

Author: fantasai

css-cascade-5/Overview.bs

@@ -1442,11 +1442,11 @@ Un-Named Layers</h5>
 
 			/* bootstrap.css */
 			/* the internal names are hidden from access, subsumed in "base" */
-			@layer base url(bootstrap-base.css);
+			@import url(bootstrap-base.css) layer(base);
 
 			/* author.css */
 			/* author has access to bootstrap.base layer, but not into unnamed layers */
-			@layer bootstrap url(bootstrap.css);
+			@import url(bootstrap.css) layer(bootstrap);
 		</pre>
 	</div>