message: docs: update gRPC-JSON example to match proto
  The `services:` array config is confusing if someone is trying to follow this documentation. Updated the line to match the above `Greeter` `proto`.
diff: @@ -121,7 +121,7 @@ gRPC or RESTful JSON requests to localhost:51051.
  - name: envoy.grpc_json_transcoder
  config:
  proto_descriptor: "/tmp/envoy/proto.pb"
  - services: ["HelloWorld"]
  + services: ["helloworld.Greeter"]
  print_options:
  add_whitespace: true
  always_print_primitive_fields: true
  