input: Write a function to count the number of vowels in a given string.
  assert num_vowels("hello") == 2
  assert num_vowels("world") == 1
  assert num_vowels("AEIOUaeiou") == 10
output: def num_vowels(s):
      vowels = "aeiouAEIOU"
      count = 0
      for char in s:
          if char in vowels:
              count += 1
      return count