input: Write a function to format a string by replacing "--" with a space.
  assert format_string("hello--world") == "hello world"
  assert format_string("this--is--a--test") == "this is a test"
  assert format_string("no--double--dashes--here") == "no double dashes here"
output: def format_string(s):
      return s.replace("--", " ")