Update helloworld-tutorial.md (#1057)

grpcurl command have been run successfully with modifieded like above.

examples/helloworld-tutorial.md

@@ -243,7 +243,7 @@ If you have a gRPC GUI client such as [Bloom RPC] you should be able to send req
 
 Or if you use [grpcurl] then you can simply try send requests like this:
 ```
-$ grpcurl -plaintext -import-path ./proto -proto helloworld.proto -d '{"name": "Tonic"}' [::]:50051 helloworld.Greeter/SayHello
+$ grpcurl -plaintext -import-path ./proto -proto helloworld.proto -d '{"name": "Tonic"}' '[::]:50051' helloworld.Greeter/SayHello
 ```
 And receiving responses like this:
 ```