input: Write a python function to count the number of vowels in a given word.
  assert count_vowels('amazing') == 3
  assert count_vowels('hello') == 2
  assert count_vowels('Python') == 1
output: def count_vowels(word):
      vowels = 'aeiouAEIOU'
      return sum(1 for char in word if char in vowels)