input: Write a python function to find the nth occurrence of a substring in a given string.
  assert find_nth_occurrence("Hello World", "l", 3) == 9
  assert find_nth_occurrence("banana", "na", 2) == 4
  assert find_nth_occurrence("abcabcabc", "abc", 3) == 6
output: def find_nth_occurrence(string, substring, n):
      count = 0
      index = -1
      while count < n:
          index = string.find(substring, index + 1)
          if index == -1:
              return -1
          count += 1
      return index