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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

4ad/go,vsekhar/elastic-go,linux-on-ibm-z/go,cloudflare/go,linux-on-ibm-z/go,polyverse-security/go,josharian/go,CAFxX/go,polyverse-security/go,suncycheng/go,theif519/go,christopher-henderson/Go,4ad/go,akutz/go,cloudflare/go,Cofyc/go,goodlang/good,golang/go,codestation/go,ImJasonH/go,akutz/go,CAFxX/go,suncycheng/go,vsekhar/elastic-go,akutz/go,theif519/go,ganboing/go-esx,derekmarcotte/go,josharian/go,suncycheng/go,4ad/go,akutz/go,suncycheng/go,Cofyc/go,ganboing/go-esx,derekmarcotte/go,ImJasonH/go,josharian/go,polyverse-security/go,nawawi/go,vsekhar/elastic-go,akutz/go,josharian/go,CAFxX/go,suncycheng/go,ganboing/go-esx,Cofyc/go,derekmarcotte/go,ImJasonH/go,momchil-velikov/go,goodlang/good,4ad/go,Cofyc/go,vsekhar/elastic-go,golang/go,CAFxX/go,goodlang/good,golang/go,vsekhar/elastic-go,nawawi/go,momchil-velikov/go,linux-on-ibm-z/go,Cofyc/go,codestation/go,derekmarcotte/go,linux-on-ibm-z/go,ganboing/go-esx,josharian/go,golang/go,codestation/go,linux-on-ibm-z/go,polyverse-security/go,derekmarcotte/go,theif519/go,ImJasonH/go,golang/go,josharian/go,theif519/go,suncycheng/go,momchil-velikov/go,theif519/go,ganboing/go-esx,momchil-velikov/go,Cofyc/go,cloudflare/go,linux-on-ibm-z/go,golang/go,josharian/go,cloudflare/go,polyverse-security/go,cloudflare/go,akutz/go,CAFxX/go,akutz/go,cloudflare/go,theif519/go,linux-on-ibm-z/go,codestation/go,nawawi/go,momchil-velikov/go,polyverse-security/go,codestation/go,vsekhar/elastic-go,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,4ad/go,codestation/go,nawawi/go,4ad/go,momchil-velikov/go,theif519/go,ganboing/go-esx,nawawi/go,4ad/go,nawawi/go,josharian/go,ImJasonH/go,golang/go,derekmarcotte/go,ganboing/go-esx,ImJasonH/go,goodlang/good,akutz/go,Cofyc/go,vsekhar/elastic-go,goodlang/good,CAFxX/go,goodlang/good,linux-on-ibm-z/go,codestation/go,nawawi/go,golang/go,momchil-velikov/go,ImJasonH/go,codestation/go,suncycheng/go,cloudflare/go,polyverse-security/go,CAFxX/go,4ad/go,polyverse-security/go,momchil-velikov/go,goodlang/good,CAFxX/go,cloudflare/go,ImJasonH/go,suncycheng/go,ganboing/go-esx,goodlang/good,nawawi/go,derekmarcotte/go,theif519/go