arrow

MINOR: [C++] Fix the wrong comment in #45104 (#45111)



### Rationale for this change

In my last typo fix in #45104 I made a mistake: https://github.com/apache/arrow/pull/45104/files#diff-a1f24396d20cd510ae48838e684fbc5ba7b23475702ae849a2d13bf48e5251aaR284 I was too careless. The size of a block in swiss table is much more subtle than I thought. It turns out the original `12KB` was right.

### What changes are included in this PR?

Removing my last wrong change and elaborate a bit about how the number `12KB` comes.

### Are these changes tested?

### Are there any user-facing changes?

Authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>