input: Write a python function to check if one string is a substring of another and return True if it is, else return False.
  assert string_match("hello", "ell")==True
  assert string_match("testing", "ing")==True
  assert string_match("world", "word")==False
  assert string_match("python", "thon")==True
output: def string_match(main_string, sub_string):
      return sub_string in main_string