update prob_dist

Author: longye-tian

lectures/prob_dist.md

@@ -289,21 +289,13 @@ The mean and variance are:
 ```{code-cell} ipython3
 λ = 2
 u = scipy.stats.poisson(λ)
-```
-    
-```{code-cell} ipython3
 u.mean(), u.var()
 ```
-
-The the expectation of Poisson distribution is $\lambda$ and the variance is also $\lambda$.
+    
+The expectation of Poisson distribution is $\lambda$ and the variance is also $\lambda$.
 
 Here's the PMF:
 
-```{code-cell} ipython3
-λ = 2
-u = scipy.stats.poisson(λ)
-```
-
 ```{code-cell} ipython3
 u.pmf(1)
 ```