Make code example more realistic and safe

1. Without a real sorting part, the data output is not the same as with real sort (as the array is not really sorted when the callback returns `undefined`).
2. Current example may somehow encourage a reader to write sorting callbacks with undetermined behavior.

1-js/05-data-types/05-array-methods/article.md

@@ -426,6 +426,7 @@ By the way, if we ever want to know which elements are compared -- nothing preve
 ```js run
 [1, -2, 15, 2, 0, 8].sort(function(a, b) {
   alert( a + " <> " + b );
+  return a - b;
 });
 ```