chore: fix math formula.

Author: fuookami

README.md

@@ -73,7 +73,7 @@ y \leq \sum_{i} x_{i}, \\; \forall i
 $$
 
 $$
-y \in \{ 0, 1 \}
+y \in \\{ 0, 1 \\}
 $$
 
 Of course, you can also extend these intermediate expressions according to your own business requirements. At this point, you need to implement some interfaces to let ospf know which intermediate variables and constraints need to be added for these intermediate expressions.

README_ch.md

@@ -72,7 +72,7 @@ y \leq \sum_{i} x_{i}, \\; \forall i
 $$
 
 $$
-y \in \{ 0, 1 \}
+y \in \\{ 0, 1 \\}
 $$
 
 当然，你也可以根据自己的业务需求，拓展这些函数中间值。这个时候你要实现一些接口，以让 ospf 知道这个函数中间值需要添加哪些中间变量以及约束。