As a Muslim it is wajib on us to pay respect to quran that it is book of Allah. But kaffarah applies when some one made mistake and that is intentionally or willingly.
As it was mentioned it was mistakenly that means it was not an intentional act
But better to say astaghferullaha
We must pay respect to whole quran or page of quran or aayat quran.
It has same status because it is part of the quran