doc: use PBKDF2 in text

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

PR-URL: #18279
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Vladimir Kurchatkin <vladimir.kurchatkin@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: tniessen

doc/api/crypto.md

@@ -1250,7 +1250,7 @@ password always creates the same key. The low iteration count and
 non-cryptographically secure hash algorithm allow passwords to be tested very
 rapidly.
 
-In line with OpenSSL's recommendation to use pbkdf2 instead of
+In line with OpenSSL's recommendation to use PBKDF2 instead of
 [`EVP_BytesToKey`][] it is recommended that developers derive a key and IV on
 their own using [`crypto.pbkdf2()`][] and to use [`crypto.createCipheriv()`][]
 to create the `Cipher` object. Users should not use ciphers with counter mode
@@ -1312,7 +1312,7 @@ password always creates the same key. The low iteration count and
 non-cryptographically secure hash algorithm allow passwords to be tested very
 rapidly.
 
-In line with OpenSSL's recommendation to use pbkdf2 instead of
+In line with OpenSSL's recommendation to use PBKDF2 instead of
 [`EVP_BytesToKey`][] it is recommended that developers derive a key and IV on
 their own using [`crypto.pbkdf2()`][] and to use [`crypto.createDecipheriv()`][]
 to create the `Decipher` object.