input: Write a function to generate a list of even numbers within a specified range.
  assert generate_even_numbers(1, 10) == [2, 4, 6, 8, 10]
  assert generate_even_numbers(3, 15) == [4, 6, 8, 10, 12, 14]
  assert generate_even_numbers(10, 20) == [10, 12, 14, 16, 18, 20]
output: def generate_even_numbers(start, end):
      return [num for num in range(start, end + 1) if num % 2 == 0]