utils: fix whitespace in Pagure error message
    
    Prior to this change, Python would concatenate the two quoted strings
    together without a space, and fedpkg's error message would read
    
      "... to set atoken in your user configuration."
    
    Add a space between "a" and "token".
    
    Signed-off-by: Ken Dreyer <kdreyer@redhat.com>