Update signing example with valid System.time_unit

[matt-glover]

I was running through the tutorial and wiring up an example of the in-app notifications. When I hit the signing section I copy/pasted the example code from the README and got an error:

 ** (ArgumentError) unsupported time unit. Expected :day, :hour, :minute, :second, :millisecond, :microsecond, :nanosecond, or a positive integer, got :seconds
        (elixir 1.15.7) lib/calendar/datetime.ex:1621: DateTime.add/4

... TRUNCATED...

I'm running Elixir 1.8+ where :seconds was hard deprecated.

This PR adjusts the DateTime parameter in the docs to :second which is accepted by the add function in the latest versions of Elixir (I think since 1.4?).

[brentjanderson]

Thank you for addressing this! We'll get this merged and out shortly.