fix: fix name

    - 概率与期望: math/expectation.md

    - 置换群: math/permutation-group.md

    - 高斯消元: math/gauss.md

    - xor 模线性方程组: math/linear-equations.md

    - 线性方程组: math/linear-equations.md

    - 容斥原理: math/inclusion-exclusion-principle.md

    - 抽屉原理: math/drawer-principle.md

    - 数值积分: math/integral.md