Update env var name in readme

[antn]

hey @bhuga! I was setting this up, and noticed that the env variable for the private key seems to be RPC_PRIVATE_KEY, not CHATOPS_PRIVATE_KEY:

hubot-chatops-rpc/src/chatops-rpc.coffee

Lines 35 to 38 in 20a6192

	privateKey = process.env.RPC_PRIVATE_KEY
	unless privateKey?
	robot.logger.error "The RPC_PRIVATE_KEY environment variable is not set; RPC commands are unknown."
	return

This PR updates the README to use the correct name for the env variable 🙇‍♀️

[bhuga]

Woops, thanks!