crypto/pbkdf2: fix code example for Key
The type for password is not `[]byte` (as it was in golang.org/x/crypto/pbkdf2), it is `string`.
Change-Id: I914a81a500a6d93f994b587814f26285aef7b96d
GitHub-Last-Rev:
5ec752e0def59c1058c649b4543f296467691813
GitHub-Pull-Request: golang/go#72746
Reviewed-on: https://go-review.googlesource.com/c/go/+/656115
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Sean Liao <sean@liao.dev>
Auto-Submit: Sean Liao <sean@liao.dev>