fix typo in trainer tutorial
fix typo in trainer tutorial

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