input: Write a python function to convert numbers to their word equivalents using a dictionary.
  assert num_to_str(1) == 'one'
  assert num_to_str(5) == 'five'
  assert num_to_str(10) == 'ten'
  assert num_to_str(11) == 'invalid number'
output: def num_to_str(num):
      num_dict = {
          1: 'one',
          2: 'two',
          3: 'three',
          4: 'four',
          5: 'five',
          6: 'six',
          7: 'seven',
          8: 'eight',
          9: 'nine',
          10: 'ten'
      }
      return num_dict.get(num, 'invalid number')