Incorrect documentation for scheduling

[HarshBalyan]

Readme -> Scheduling

# 10 seconds
schedule_time = 10_000

Flume.enqueue_in(:queue_name, schedule_time, MyApp.FancyWorker, [arg_1, arg_2])

Here the schedule_time is shown to be in milliseconds. It should actually be unix time in seconds.

Also, Flume.enqueue_in/4, Flume.enqueue_in/5 and Flume.enqueue_in/6 accept argument time_in_seconds. It is confusing. A better name for this argument would be unix_time_in_seconds.