Fix typo in wavenet example requirements
Fix typo in wavenet example requirements

pfnet/chainer,okuta/chainer,wkentaro/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,okuta/chainer,niboshi/chainer,wkentaro/chainer,wkentaro/chainer,chainer/chainer,okuta/chainer,okuta/chainer,niboshi/chainer,chainer/chainer,hvy/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,wkentaro/chainer