input: Write a python function to check if one string is a substring of another.
  assert is_substring("hello world", "world") == True
  assert is_substring("hello world", "python") == False
  assert is_substring("abcdef", "def") == True
  assert is_substring("abcdef", "gh") == False
output: def is_substring(main_string, sub_string):
      return sub_string in main_string