message: Fix an example in GOLANG-HOWTO.md
  In the example of preparation to use hybrid decryption, it appears that the variables declared as `&keyset.MemReaderWriter{}` are not correctly referenced in the following marshaling function.
diff: @@ -286,7 +286,7 @@ func main() {
  return nil, err
  }
  - ksPriv, err := proto.Marshal(exported.Keyset)
  + ksPriv, err := proto.Marshal(exportedPriv.Keyset)
  if err != nil {
  return nil, err
  }
  @@ -306,7 +306,7 @@ func main() {
  return nil, err
  }
  - ksPub, err := proto.Marshal(exported.Keyset)
  + ksPub, err := proto.Marshal(exportedPub.Keyset)
  if err != nil {
  return nil, err
  }
  