input: Write a python function to count the number of vowels in a given word.
  assert count_vowels("hello")==2
  assert count_vowels("world")==1
  assert count_vowels("python")==1
  assert count_vowels("aeiou")==5
output: def count_vowels(word):
      vowels = "aeiouAEIOU"
      return sum(1 for char in word if char in vowels)