input: Write a python function to concatenate a list of strings.
  assert concatenate_strings(['hello', ' ', 'world']) == 'hello world'
  assert concatenate_strings(['Python', ' is ', 'awesome']) == 'Python is awesome'
  assert concatenate_strings(['abc', 'def', 'ghi']) == 'abcdefghi'
output: def concatenate_strings(strings):
      return ''.join(strings)