練習存取二維陣列中的元素：

num_array = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
]
x = num_array[0] # x會變成 [1, 2, 3]
y = num_array[0][2] # y會變成3
num_array[2][1] = -8 # num_array[2][1]由8變成-8
print( x )
print( y )
print(num_array[2][1])