input: Write a python function to check if a substring is present in a given string.
  assert search_substring('hello world', 'world') == True
  assert search_substring('hello world', 'python') == False
  assert search_substring('abcdef', 'abc') == True
  assert search_substring('abcdef', 'defg') == False
output: def search_substring(main_string, sub_string):
      return sub_string in main_string