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: #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>

doc/api/crypto.md

@@ -1395,7 +1395,7 @@ higher the number of iterations, the more secure the derived key will be,
 but will take a longer amount of time to complete.
 
 The `salt` should also be as unique as possible. It is recommended that the
-salts are random and their lengths are greater than 16 bytes. See
+salts are random and their lengths are at least 16 bytes. See
 [NIST SP 800-132][] for details.
 
 Example:
@@ -1429,7 +1429,7 @@ higher the number of iterations, the more secure the derived key will be,
 but will take a longer amount of time to complete.
 
 The `salt` should also be as unique as possible. It is recommended that the
-salts are random and their lengths are greater than 16 bytes. See
+salts are random and their lengths are at least 16 bytes. See
 [NIST SP 800-132][] for details.
 
 Example: