input: Write a python function to replace each word in a sentence with its first letter.
  assert replace_with_first_letter("Hello World") == "HW"
  assert replace_with_first_letter("OpenAI creates artificial intelligence") == "Ocai"
  assert replace_with_first_letter("Python is fun") == "Pif"
output: def replace_with_first_letter(sentence):
      return ''.join(word[0] for word in sentence.split())