Display correct completed_at timestamp in details

Completed jobs would show a relative time value that matched how long
the job took rather than when it completed.

Closes #137

Author: sorentwo

lib/oban/web/live/jobs/timeline_component.ex

@@ -104,7 +104,7 @@ defmodule Oban.Web.Jobs.TimelineComponent do
 
       {"executing", "completed", at} ->
         words =
-          at
+          job.completed_at
           |> DateTime.diff(now)
           |> Timing.to_words()