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