input: Write a function to format a string to a specified length using ljust.
  assert format_string("Hello", 10) == "Hello     "
  assert format_string("Data", 8) == "Data    "
  assert format_string("AI", 5) == "AI   "
  assert format_string("OpenAI", 10) == "OpenAI    "
output: def format_string(s, length):
      return s.ljust(length)