Update the bookstore example.

## In which file did you encounter the issue?

https://github.com/GoogleCloudPlatform/python-docs-samples/tree/master/endpoints/bookstore-grpc-transcoding

## Describe the issue

We got a bug report for https://cloud.google.com/apis/design/ (internal bug number: 148977958), claiming that the example protos here are inconsistent with the standard methods design guidelines in the style guide. The user claims not to know which is authoritative.

I checked, and this is true, and it would be nice if the sample here was canonical (although I expect that to be low priority, natch).