ch2_python_intro.md 中集合的定义有错

这里t_set = (1, 3, 5 )，应该是{1, 3, 5}
如果定义一个列表，用set方法转成集合后，也确实是{xxx,xxx,xxx}，而不是 ( )

Open ch2_python_intro.md and locate the t_set example. Check the surrounding explanation of Python sets, then correct the example to use set syntax and ensure the text consistently describes the resulting value. Done means the example and explanation agree.

Written by the indexing model from the issue text.