input: Write a function to display key-value pairs of a dictionary.
  assert display_dict({'a': 1, 'b': 2, 'c': 3}) == "a: 1\nb: 2\nc: 3\n"
  assert display_dict({'apple': 'fruit', 'carrot': 'vegetable'}) == "apple: fruit\ncarrot: vegetable\n"
  assert display_dict({}) == ""
output: def display_dict(d):
      result = ""
      for key, value in d.items():
          result += f"{key}: {value}\n"
      return result