doc: use PBKDF2 in text

Use upper case variant in text in compliance with RFC 2898.

PR-URL: https://github.com/nodejs/node/pull/18279


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>