Correct default backoff explanation in Worker

Closes #1110

lib/oban/worker.ex

@@ -123,8 +123,8 @@ defmodule Oban.Worker do
   When jobs fail they may be retried again in the future using a backoff algorithm. By default the
   backoff is exponential with a fixed padding of 15 seconds and a small amount of jitter. The
   jitter helps to prevent jobs that fail simultaneously from consistently retrying at the same
-  time. With the default backoff behavior, the 20th attempt will occur around 12 days after the
-  first attempt.
+  time. With the default backoff behavior the 20th attempt will occur around 12 days after the
+  19th attempt, and a total of 25 days after the first attempt.
 
   If the default strategy is too aggressive or otherwise unsuited to your app's workload you can
   define a custom backoff function using the `c:backoff/1` callback.