input: Write a function to find a substring in a given string and return the start index of the substring. If the substring is not found in the string, return -1.
  assert find_substring("hello world", "world") == 6
  assert find_substring("hello world", "hello") == 0
  assert find_substring("hello world", "planet") == -1
  assert find_substring("hello world", "or") == 7
  assert find_substring("hello world", "o") == 4
output: def find_substring(s, sub):
      index = s.find(sub)
      return index