input: Write a function to find the most frequent word in a sentence.
  assert freq_word("this is a test this is only a test") == "test"
  assert freq_word("word word word test test") == "word"
  assert freq_word("a b c d e f g h i j k l m n o p q r s t u v w x y z z z z") == "z"
output: def freq_word(sentence):
      words = sentence.split()
      word_count = {}
      for word in words:
          if word in word_count:
              word_count[word] += 1
          else:
              word_count[word] = 1
      most_frequent_word = max(word_count, key=word_count.get)
      return most_frequent_word