input: Write a python function to print the keys and values of a dictionary in the format "key -> value".
  assert print_dict({1:'one', 2:'two', 3:'three'})==None
  assert print_dict({4:'four', 5:'five'})==None
  assert print_dict({6:'six', 7:'seven'})==None
output: def print_dict(d):
      for key, value in d.items():
          print(f"{key} -> {value}")