Paddle-Inference-Demo，这行代码错了吧

[lovychen]

Paddle-Inference-Demo/python/advanced/multi_thread/threads_demo.py

Line 104 in cbf3a52

for i in range(thread_num):

是不是应该是：
for t in threads：
t.join()

[vivienfanghuagood]

Paddle-Inference-Demo/python/advanced/multi_thread/threads_demo.py

Line 104 in cbf3a52

for i in range(thread_num):

是不是应该是： for t in threads： t.join()

谢谢指出错误，我们下个pr修复。