Update poll step_function param description

Changes code example to accurately demonstrate the text of the param description. The description is:

As an example, you can increase the wait time between calling the target function by 10 seconds every iteration until the step is 100 seconds--at which point it should remain constant at 100 seconds

If step reaches 100, then step += 10 becomes 110, and max(110, 100) == 110, so 110 is returned as the next step.

Changing max to min will make the code example accurate.

PS: Thanks for the great library, it's been super useful!

ddmee / polling2

Update poll step_function param description #74