doc: modernize and fix code examples in util.md

* Remove useless constructor.

* Use template literals.

* Update code example.
  Now all arrays with just holes are outputted the same way.
  In the fixed example, it was `[ <101 empty items> ]` twice.

PR-URL: https://github.com/nodejs/node/pull/13298


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>