input: Write a python function to convert a list of integers to a string with each integer separated by a comma.
  assert listToString([1, 2, 3, 4, 5]) == '1,2,3,4,5'
  assert listToString([10, 20, 30, 40]) == '10,20,30,40'
  assert listToString([7, 8, 9, 10]) == '7,8,9,10'
output: def listToString(lst):
      return ','.join(map(str, lst))