input: Write a python function to check if a string contains a substring.
  assert contains_substring("This is a string", "is") == True
  assert contains_substring("This is a string", "xyz") == False
  assert contains_substring("Hello, World!", "World") == True
  assert contains_substring("Hello, World!", "Python") == False
output: def contains_substring(main_string, substring):
      return substring in main_string