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