update examples to use addService

[mitchdraft]

small change to the samples now that the node server api has changed

link to api: https://github.com/grpc/grpc-node/blob/master/packages/grpc-native-core/src/server.js#L948

the examples work with or without this change, but the change removes this warning message:

$ node server.js
(node:44083) DeprecationWarning: Server#addProtoService: Use Server#addService instead

complimentary PR submitted on docs repo:
grpc/grpc.github.io#776

[stanley-cheung]

Thanks for the fix!