Retry block example in README is incorrect

[justinlittman]

In particular, it references retries_remaining instead of retry_count in /~https://github.com/lostisland/faraday-retry#call-a-block-on-every-retry

Should be:

response_statuses = []
retry_options = {
  retry_block: -> (env:, options:, retry_count:, exception:, will_retry_in:) { response_statuses << env.status }
}

[olleolleolle]

@justinlittman Cool! Would you like to make a PR?