Hi @Haiyun_Hu. As @TMosh mentioned it was a mistake on the slides, and it is being fixed.
However, I did want to point out that in the code you attached you seem to be using the t distribution to compute the probability. That’s the case when the variance (or standard deviation) is unknown. In this particular example the standard deviation has a value of 3, so the statistic actually follows a Gaussian distribution, and not a t.