input: Write a python function to split a sentence into words using the split function.
  assert split_into_words("Hello world") == ["Hello", "world"]
  assert split_into_words("The quick brown fox") == ["The", "quick", "brown", "fox"]
  assert split_into_words("Python is fun") == ["Python", "is", "fun"]
output: def split_into_words(sentence):
      return sentence.split()