input: Write a python function to convert a list of words into a single string where each word is separated by a comma and a space.
  assert listToString(['apple', 'banana', 'cherry']) == 'apple, banana, cherry'
  assert listToString(['dog', 'cat', 'mouse']) == 'dog, cat, mouse'
  assert listToString(['hello', 'world']) == 'hello, world'
output: def listToString(words):
      return ', '.join(words)