fix gRPC README in examples

[wapa5pow]

https://github.com/census-instrumentation/opencensus-go/blob/master/examples/grpc/helloworld_server/main.go#L52

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[wapa5pow]

I signed it!

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CLAs look good, thanks!

[rakyll]

Thanks!

[rakyll]

Fixing the flaky CI, see #629.