crypto/aes: fix key size typo

crypto/aes: fix key size typo

AES-196 does not exist, but AES-192 does.

Change-Id: Iecdcae28fde807d148af9d09c7291fc02c9f6edd
Reviewed-on: https://go-review.googlesource.com/c/go/+/399495
Reviewed-by: Filippo Valsorda <filippo@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Filippo Valsorda <filippo@golang.org>