update node docs to use addService

[mitchdraft]

small change to the docs 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

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[mitchdraft]

CLA signed

[carl-mastrangelo]

Change seems fine, @murgatroid99 & @ZhouyihaiDing can you tell me if I should merge?

[ZhouyihaiDing]

The change seems fine to me too. @murgatroid99 can you please take a look?

[hsaliak]

@murgatroid99 @nicolasnoble can you please rubber stamp, given that this is an example change it would be best if one of you looked over it.

[thisisnotapril]

@murgatroid99 @nicolasnoble @hsaliak are we OK to merge this one?