crypto/cipher: use AEAD.NonceSize to make nonce in the example
The existing example uses hard-coded constant to make nonce buffer.
Using AEAD.NonceSize makes it a more portable and appropriate example.
Fixes: #48372
Change-Id: I7c7a38ed48aff46ca11ef4f5654c778eac13dde6
GitHub-Last-Rev:
03ccbb16df4ca9cbd4a014836aee0f54b2ff3002
GitHub-Pull-Request: golang/go#48373
Reviewed-on: https://go-review.googlesource.com/c/go/+/349603
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Sean Liao <sean@liao.dev>
Reviewed-by: Roland Shoemaker <roland@golang.org>