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.

Author: vsemozhetbyt

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

@@ -427,6 +427,7 @@ alert(arr);  // *!*1, 2, 15*/!*
 ```js run
 [1, -2, 15, 2, 0, 8].sort(function(a, b) {
   alert( a + " <> " + b );
+  return a - b;
 });
 ```