modified example in README.md

Clarified arguments of critSecDone callback.

Currently README.md vaguely specifies that "Since the release function is itself asynchronous, you can pass it a callback that will be invoked when the release has been confirmed". This doesn't mentions that callback to the release function should be retrieved as a third argument, as the second is the retries count. Previously, this could be inferred only from the source code, in this pull request I've modified the example in README.md to show this explicitly.