input: Write a python function to find the most frequent word in a given string.
  assert most_frequent_word("This is an example of a text with some words") == "a"
  assert most_frequent_word("hello world hello") == "hello"
  assert most_frequent_word("one two three two three three") == "three"
output: def most_frequent_word(s):
      words = s.split()
      frequency = {}
      for word in words:
          if word in frequency:
              frequency[word] += 1
          else:
              frequency[word] = 1
      most_frequent = max(frequency, key=frequency.get)
      return most_frequent