doc: make mkdtemp example work on Windows

PR-URL: https://github.com/nodejs/node/pull/15408
Fixes: https://github.com/nodejs/node/issues/14960


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>