Fix incorrect currentTimeMillis() call

[adamszadkowski]

I have made those changes to fix issue in here: https://kotlinlang.org/docs/reference/coroutines/flow.html

Problem:

[elizarov]

Good catch, thanks. But changes shall be applied to sources in docs/flow.md file, not to the auto-generated .kt file (which are actually correct). I've fixed it. See #1496