Fix: 2 typos in variant section

[YukunJ]

Brief: Fix 2 typos in std::variant section.

1. int type usually takes 4 bytes on 64 bit architectures.
2. std::variant<int, std::string> would take 8 (sizeof(size_t)) + 32 (max(sizeof(int), sizeof(std::string)) storage space then.

Please refer to this for reproduction: https://godbolt.org/z/sbvEMW56G

@Charliechen114514