Yep, I've done this a few times..!

If the nick has resulted in a flap of leather hanging off, you can glue that down with rubber cement and then sand it with fine paper to smooth it off. Use 1200/1500 grit, something like that!

If its just a slice then you can just sand it down to smooth off the slice.

Sorry for the two bits of advice, but I couldnt see the picture..!