Update protocol spec to align with implementation

[CVTJNII]

The following pull updates the protocol spec with the following changes:

• Update version to 2 as the JSON frame is a v2 improvement per https://discuss.elastic.co/t/logstash-beats-input-lumberjack-protocol-v1-support-lifetime/56334
• Note that the version byte is ASCII per

  logstash-input-beats/lib/lumberjack/beats/server.rb

  Lines 219 to 220 in a2ad0ea

  	PROTOCOL_VERSION_1 = "1".ord
  	PROTOCOL_VERSION_2 = "2".ord

• Note that the window frame size needs to be set before each frame of messages, found by troubleshooting reset_next_ack() errors while implementing the spec.

Not sure if the CLA is required for simple documentation fixes, so please let me know if it is. Thanks.

[untergeek]

CLA is necessary for even documentation changes, yes. Please sign the CLA.

[CVTJNII]

Okay, I'll work on getting it signed. I need corporate approval so it will probably take a little bit. Thanks.

[ph]

@CVTJNII Changes LGTM, so I will gladly merge it when the CLA is signed.