Fix capitalization of `F.relu` in doc
Fix capitalization of `F.relu` in doc

wkentaro/chainer,pfnet/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,wkentaro/chainer,niboshi/chainer,niboshi/chainer,niboshi/chainer,hvy/chainer,chainer/chainer,keisuke-umezawa/chainer,wkentaro/chainer,chainer/chainer,keisuke-umezawa/chainer,keisuke-umezawa/chainer,wkentaro/chainer,okuta/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,hvy/chainer,chainer/chainer,okuta/chainer,chainer/chainer