The first character isn't a dash "-" it is a tilde "~" (on my US keyboard is is left of "1").
If you are already in your pi home directory (it will look something like "pi@openplotter:~ $") just type "nano .config/openplotter/openplotter.conf".
If you are already in your pi home directory (it will look something like "pi@openplotter:~ $") just type "nano .config/openplotter/openplotter.conf".