Fix the document of the gpu section in the tutorial
Fix the document of the gpu section in the tutorial

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