Fix AsyncCountdownIter example in docs/devel.rst

[justinmoon]

This is how the example should run:

>>> run(main)
T-minus 5
T-minus 4
T-minus 3
T-minus 2
T-minus 1

Prior to this fix, it stops at 2:

>>> run(main)
T-minus 5
T-minus 4
T-minus 3
T-minus 2