Update time_complexity.md

[hopkings2008]

• correct the implement of exp_recur function and remove +1 operation from the function to simulate the cell division process

[krahets]

This function represents the “number of divisions” of the cells after n generations, not the "number of cells after" n generations.

For the latter, it should be:

/* 指数阶（递归实现） */
fn exp_recur(n: i32) -> i32 {
    if n == 0 {  # 
        return 1;
    }
    exp_recur(n - 1) + exp_recur(n - 1)
}

I will add a description to clarify this and avoid ambiguity.