Update max_product_cutting_problem.md

[sunshinesDL]

本节结尾的正确性证明中，

使用反证法，只分析 n >= 3 的情况。

这句是否应该改为 “只分析 n >= 4 的情况”，因为 n == 3 时，由于题目要求至少将 n 切分为两个正整数，所以此时应该切分为 1 * 2 ，切分方案里是包含 1 的 😁；

此外，对于这句描述：

假设最优切分方案中存在 >= 4 的因子 x ，那么一定可以将其继续划分为 2(x-2) ，从而获得更大的乘积。

最后半句改为“从而获得更大或可替代的乘积”是否更好，因为当切分方案中存在 4 作为因子时，将其划分为 2 * 2，乘积相对划分前是相等的。不过这处改动不影响“所有因子 <= 3” 的结论，因为因子 4 是可以被替代的。

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