Update chainer/functions/math/zeta.py
Update chainer/functions/math/zeta.py

added changes

Co-Authored-By: Hiroyuki Vincent Yamazaki <a5848102d8491b6959f66fb5d5231fc8bd6a56a0@gmail.com>
chainer/chainer,hvy/chainer,hvy/chainer,niboshi/chainer,chainer/chainer,wkentaro/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,hvy/chainer,chainer/chainer,pfnet/chainer,niboshi/chainer,chainer/chainer,niboshi/chainer,wkentaro/chainer,niboshi/chainer