broken link - PgpSignedAttestation.md

[PgpSignedAttestation.md](https://github.com/Grafeas/Grafeas/blob/master/samples/server/go-server/api/docs/PgpSignedAttestation.md) referenced as part of [#create-a-pgpsignedattestation-occurrence](https://github.com/kelseyhightower/grafeas-tutorial#create-a-pgpsignedattestation-occurrence) section in read me is broken.