crypto/pbkdf2: update RFC reference in package doc

Now that it's published, we don't need to explain how we diverge from
the old one.

Change-Id: If2c22e89dd1b9fc531a363b5fb7b1eb5720eb84e
Reviewed-on: https://go-review.googlesource.com/c/go/+/637215
Auto-Submit: Filippo Valsorda <[email protected]>
Reviewed-by: Daniel McCarney <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Roland Shoemaker <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

src/crypto/pbkdf2/pbkdf2.go

@@ -2,20 +2,12 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-/*
-Package pbkdf2 implements the key derivation function PBKDF2 as defined in RFC
-2898 / PKCS #5 v2.0.
-
-A key derivation function is useful when encrypting data based on a password
-or any other not-fully-random data. It uses a pseudorandom function to derive
-a secure encryption key based on the password.
-
-While v2.0 of the standard defines only one pseudorandom function to use,
-HMAC-SHA1, the drafted v2.1 specification allows use of all five FIPS Approved
-Hash Functions SHA-1, SHA-224, SHA-256, SHA-384 and SHA-512 for HMAC. To
-choose, you can pass the `New` functions from the different SHA packages to
-pbkdf2.Key.
-*/
+// Package pbkdf2 implements the key derivation function PBKDF2 as defined in
+// RFC 8018 (PKCS #5 v2.1).
+//
+// A key derivation function is useful when encrypting data based on a password
+// or any other not-fully-random data. It uses a pseudorandom function to derive
+// a secure encryption key based on the password.
 package pbkdf2
 
 import (