Modify README.md of chap03

[bestxmg]

I think it may be inexact in the modified line of this statement: vector<vector> ivec;
1: In the chapter 3.3(page 97), it says in the book that the definition of a vector to a vector will vary on different C++ version. I had tried it on my machine and verified it is right.
2: This statement is a default initialization (page 43), not a value initialization (page 132).
In C++, value-initializing a std::vector means creating a vector where its elements are explicitly initialized to their default values. For example:
std::vector<int> vec(5); // Initializes 5 elements, all set to 0 (value-initialized for int)

[bestxmg]