scrypt: fix broken example in documentation

Fixes golang/go#17046

Change-Id: I7fa7e0b700212992125de32524801048b56ac5d1
Reviewed-on: https://go-review.googlesource.com/28952
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

scrypt/scrypt.go

@@ -218,7 +218,7 @@ func smix(b []byte, r, N int, v, xy []uint32) {
 // For example, you can get a derived key for e.g. AES-256 (which needs a
 // 32-byte key) by doing:
 //
-//      dk := scrypt.Key([]byte("some password"), salt, 16384, 8, 1, 32)
+//      dk, err := scrypt.Key([]byte("some password"), salt, 16384, 8, 1, 32)
 //
 // The recommended parameters for interactive logins as of 2009 are N=16384,
 // r=8, p=1. They should be increased as memory latency and CPU parallelism