input: Write a python function to print a message based on a single command-line argument.
  assert check_argument(['hello'])=='You entered hello.'
  assert check_argument([])=='Please provide a single argument.'
  assert check_argument(['one', 'two'])=='Please provide a single argument.'
output: def check_argument(args):
      if len(args) == 1:
          return f'You entered {args[0]}.'
      else:
          return 'Please provide a single argument.'