input: Write a python function to join a list of strings into a single string with spaces in between.
  assert join_list(["hello", "world"]) == "hello world"
  assert join_list(["this", "is", "a", "test"]) == "this is a test"
  assert join_list(["join", "these", "words"]) == "join these words"
output: def join_list(strings):
      return " ".join(strings)