async_generator example in README

[Quuxplusone]

https://github.com/lewissbaker/cppcoro#async_generatort uses some type called "threadpool" with a member function named "delay". Even if the type were supposed to be cppcoro::static_thread_pool, static_thread_pool still doesn't have any member named "delay".

The example at the moment is:

cppcoro::async_generator<int> ticker(int count, threadpool& tp)
{
  for (int i = 0; i < count; ++i)
  {
    co_await tp.delay(std::chrono::seconds(1));
    co_yield i;
  }
}

cppcoro::task<> consumer(threadpool& tp)
{
  auto sequence = ticker(10, tp);
  for co_await(std::uint32_t i : sequence)
  {
    std::cout << "Tick " << i << std::endl;
  }
}

It would be nice if the example were modified (hopefully just slightly?) to be compileable.