Fix convolution_nd docstring.
Fix convolution_nd docstring.

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