gnomedesktop typo: clock_format instead of clock_show_format

In the documentation there is a typo in this example: http://docs.saltstack.com/en/latest/ref/states/all/salt.states.gnomedesktop.html#configuration-of-the-gnome-desktop

'clock_show_format' should be 'clock_format'

This fixes issue https://github.com/saltstack/salt/issues/13759