fix typo in roi_max_align_2d.py
fix typo in roi_max_align_2d.py

keisuke-umezawa/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer,tkerola/chainer,hvy/chainer,ktnyt/chainer,chainer/chainer,niboshi/chainer,jnishi/chainer,jnishi/chainer,ktnyt/chainer,pfnet/chainer,hvy/chainer,niboshi/chainer,hvy/chainer,niboshi/chainer,jnishi/chainer,chainer/chainer,wkentaro/chainer,jnishi/chainer,ktnyt/chainer,okuta/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,ktnyt/chainer,keisuke-umezawa/chainer,niboshi/chainer,keisuke-umezawa/chainer,okuta/chainer,keisuke-umezawa/chainer,okuta/chainer,chainer/chainer