fix usage message to ask for path to config file, not root folder

The error when you run just the keter binary incorrectly says you should put the path to the root folder, which confused me for a while. The documentation in the README is correct, but the usage message should be authoritative (I may be odd, but I only went and checked the README after reading the source, figuring out what I should have done, and then checking to see if the mistake was present in the docs as well).