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