doc: correct pbkdf2 salt length recommendation

According to the linked document: "The length of the randomly-generated
portion of the salt shall be at least 128 bits." [NIST SP 800-132]

PR-URL: https://github.com/nodejs/node/pull/17524


Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>