update the link to mitmul/chainer-handson

The documents from mitmul/chainer-handson linked in the readme have been moved to https://github.com/chainer-community/chainer-colab-notebook/tree/master/hands_on_en/chainer/begginers_hands_on and https://github.com/chainer-community/chainer-colab-notebook/tree/master/hands_on_ja/chainer/begginers_hands_on and are maintained over there, I think, so probably the links should be updated.