Fix how arguments are added to the parser
    
    - For positional arguments (`dist`), we need to specify `nargs` rather
      than `required` (and `requires` is a typo anyway).
    - Percent signs in help strings must be doubled, otherwise argparse
      tries to interpolate them (and fails).
    - Add a missing word in a help text.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>