fix formatting of probability distribution notation in lecture 15 notes for GAMES101

Author: Sophomore42

_code/computer_graphics/GAMES101/lecture15_raytracing_light_transport.md

@@ -50,7 +50,7 @@ $l(u)=e(u)+\int l(V)K(u,v)dV$
 ## Probability Review
 
 - 随机变量:X
-- 分布:$X~p(x)$
+- 分布:$X\sim p(x)$
 - 概率
 - 期望
 - 概率密度函数(Probability Distribution Function, PDF)