Fix max_pooling_nd docstring.
Fix max_pooling_nd docstring.

rezoo/chainer,jnishi/chainer,cupy/cupy,delta2323/chainer,pfnet/chainer,ktnyt/chainer,cupy/cupy,cupy/cupy,hvy/chainer,keisuke-umezawa/chainer,chainer/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,jnishi/chainer,chainer/chainer,okuta/chainer,ktnyt/chainer,okuta/chainer,okuta/chainer,aonotas/chainer,hvy/chainer,chainer/chainer,cupy/cupy,ysekky/chainer,niboshi/chainer,kashif/chainer,tkerola/chainer,jnishi/chainer,keisuke-umezawa/chainer,jnishi/chainer,wkentaro/chainer,kiyukuta/chainer,hvy/chainer,wkentaro/chainer,chainer/chainer,ktnyt/chainer,wkentaro/chainer,ktnyt/chainer,niboshi/chainer,wkentaro/chainer,niboshi/chainer,hvy/chainer,ronekko/chainer,niboshi/chainer,okuta/chainer,anaruse/chainer