input: Write a python function to filter strings that contain a given substring from a list.
  assert filter_strings(['apple', 'banana', 'apricot', 'cherry'], 'ap') == ['apple', 'apricot']
  assert filter_strings(['dog', 'cat', 'parrot', 'rabbit'], 'ra') == ['parrot', 'rabbit']
  assert filter_strings(['hello', 'world', 'hero', 'python'], 'he') == ['hello', 'hero']
output: def filter_strings(strings, substring):
      return [s for s in strings if substring in s]