fix link to ChainerMN docs in performance guide
fix link to ChainerMN docs in performance guide

chainer/chainer,chainer/chainer,pfnet/chainer,okuta/chainer,hvy/chainer,tkerola/chainer,keisuke-umezawa/chainer,niboshi/chainer,wkentaro/chainer,keisuke-umezawa/chainer,niboshi/chainer,chainer/chainer,wkentaro/chainer,hvy/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer