Incorrect callback signature provided for "client.execute" in README.md

[sarus]

Hello,

The readme states that the callback for the client.execute() method returns an error parameter and the result set:

Callback should take two arguments err and result.

but the actual callback signature is:

args.callback(executeError, executionResult, retryCount);

which includes the retryCount as a third parameter.

Would recommend updating the README with the correct callback parameters as anyone using a library that expects the last parameter to be a callback function will run into errors (e.g., if you're using the async library)