input: Write a python function to format a number to a specific number of decimal places.
  assert format_number(123.456789, 2) == "123.46"
  assert format_number(123.4, 5) == "123.40000"
  assert format_number(987.654321, 3) == "987.654"
output: def format_number(num, decimal_places):
      return f"{num:.{decimal_places}f}"