input: Write a python function to convert a list of strings into a single string.
  assert list_to_string(['hello', 'world']) == 'hello world'
  assert list_to_string(['Python', 'is', 'fun']) == 'Python is fun'
  assert list_to_string(['This', 'is', 'a', 'test']) == 'This is a test'
output: def list_to_string(lst):
      return ' '.join(lst)