crypto/pbkdf2: add missing error return value in example

This function returns two values, one was missing in the example.

Change-Id: I738597f959011260f666c29b7d3ffad8e5cdc473
GitHub-Last-Rev: 5d99e04
GitHub-Pull-Request: #76032
Reviewed-on: https://go-review.googlesource.com/c/go/+/714420
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>

Author: bystones

src/crypto/pbkdf2/pbkdf2.go

@@ -27,7 +27,7 @@ import (
 // can get a derived key for e.g. AES-256 (which needs a 32-byte key) by
 // doing:
 //
-//	dk := pbkdf2.Key(sha1.New, "some password", salt, 4096, 32)
+//	dk, err := pbkdf2.Key(sha1.New, "some password", salt, 4096, 32)
 //
 // Remember to get a good random salt. At least 8 bytes is recommended by the
 // RFC.