input: Write a python function to format a string with the proper case and punctuation.
  assert format_string("hello world") == "Hello world."
  assert format_string("PYTHON programming") == "Python programming."
  assert format_string("tHIS is a tEst") == "This is a test."
output: def format_string(s):
      s = s.lower()
      s = s.capitalize()
      if not s.endswith('.'):
          s += '.'
      return s