Update examples/mnist/README.md
Update examples/mnist/README.md

Co-Authored-By: mitmul <ab67dc6d7e426ac083c53ee8c90732f2d2a9db7b@gmail.com>
hvy/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,pfnet/chainer,wkentaro/chainer,okuta/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,tkerola/chainer,chainer/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,niboshi/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,hvy/chainer,okuta/chainer,keisuke-umezawa/chainer,chainer/chainer,wkentaro/chainer,okuta/chainer,wkentaro/chainer