doc: reduce keylen in pbkdf2 examples

PR-URL: https://github.com/nodejs/node/pull/16203
Refs: https://github.com/nodejs/node/issues/3415


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>