fix(k-alloc.cc/kalloc): remove comment about page alignment of page size

[pratikpc]

I was just looking at the course and the resources shared online.

It immensely helped me to:-

1. Reinforce my understanding of items I had studied back in the day in college or learnt during work
2. Learnt about new items that I either hadn't studied about or forgotten :-P

I quite liked the way everything is presented, with a focus on simplicity and readability which means the lectures can with some googling, be followed without having to resort to videos or notes.

However, while I was looking at the problem statement, I noticed but a minor issue with the comments in k-alloc.cc

//    If `sz` is a multiple of `PAGESIZE`, the returned pointer is guaranteed
//    to be page-aligned.

The comment could cause confusion because:-

1. kalloc's first line checks whether size is within (0, PAGESIZE)

 if (sz == 0 || sz > PAGESIZE) {
        return nullptr;
    }

4. So it sort of applies only for sz=PAGESIZE

NOTE:-

1. By all accounts, the comment is technically correct because nullptr is 0. Just confusing.
2. I believe the check was added because the code originally didn't have it, and it would have caused issues upon incrementing next_free_pa

            next_free_pa += PAGESIZE;

3. It could possibly be resolved and the check removed with a roundup logic, but while it would improve correctness, I guess the same hasn't been done to keep the code simple and more readable.