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:
5d99e04f7e1168fd0627e965508181bdb9d1ac25
GitHub-Pull-Request: golang/go#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>