crypto/cipher: execute AES-GCM decryption example

crypto/cipher: execute AES-GCM decryption example

The decryption example for AES-GCM was not executed, hiding the fact
that the provided ciphertext could not be authenticated.

This commit adds the required output comment, replaces the ciphertext
with a working example, and removes an unnecessary string conversion
along the way.

Change-Id: Ie6729ca76cf4a56c48b33fb3b39872105faa604b
Reviewed-on: https://go-review.googlesource.com/22953
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>