fix docstring for sphinx error
fix docstring for sphinx error

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