fixed typo in upgrade doc example for volatile mode changes
fixed typo in upgrade doc example for volatile mode changes

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