Update README.md with correct proxy client code example

[haosong]

The right way for proxy client setup should be:

WavefrontSender wavefrontSender = new WavefrontClient.Builder(proxyServerWithPort (tracing/metrics/histogram port)).
  messageSizeBytes(messageSizeInBytes).
  batchSize(batchSize).
  flushIntervalSeconds(flushIntervalSeconds).
  maxQueueSize(queueSize).
  build(); ```

[shavidissa]

Thank you! Updated the Readme with [1].

[1] #167