fix: correct systemd timer property name

Timers have been changed from monotonic time to realtime.

closes #1, #3

README.md

@@ -72,7 +72,7 @@ at the CLI.
    - Ensure paths are correct (ReadWriteDirectories, [WorkingDirectory](https://www.freedesktop.org/software/systemd/man/systemd.exec.html),
 [ExecStart](https://www.freedesktop.org/software/systemd/man/systemd.service.html),
 [ExecStop](https://www.freedesktop.org/software/systemd/man/systemd.service.html)).
-   - Ensure timers property "[OnUnitActiveSec](https://www.freedesktop.org/software/systemd/man/)" (backup interval) is set as
+   - Ensure timers property "[OnCalendar](https://www.freedesktop.org/software/systemd/man/)" is set as
 desired.
    ```console
    $ nano minecraft-backup.service