input: Write a function to generate a list of even numbers within a given range.
  assert generate_even_numbers(0, 5) == [0, 2, 4]
  assert generate_even_numbers(10, 20) == [10, 12, 14, 16, 18]
  assert generate_even_numbers(1, 10) == [2, 4, 6, 8]
output: def generate_even_numbers(start, end):
      return [num for num in range(start, end) if num % 2 == 0]