fix doctest failure in chainer.dataset.concat_examples
fix doctest failure in chainer.dataset.concat_examples

wkentaro/chainer,rezoo/chainer,ktnyt/chainer,jnishi/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,hvy/chainer,chainer/chainer,jnishi/chainer,okuta/chainer,okuta/chainer,hvy/chainer,chainer/chainer,wkentaro/chainer,chainer/chainer,ktnyt/chainer,wkentaro/chainer,wkentaro/chainer,okuta/chainer,hvy/chainer,ktnyt/chainer,niboshi/chainer,okuta/chainer,niboshi/chainer,keisuke-umezawa/chainer,jnishi/chainer,chainer/chainer,niboshi/chainer,pfnet/chainer,jnishi/chainer,ronekko/chainer,keisuke-umezawa/chainer,ktnyt/chainer,niboshi/chainer,hvy/chainer,tkerola/chainer,anaruse/chainer