crypto/pbkdf2: update RFC reference in package doc

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 <filippo@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>