Fix sigmoid_cross_entropy doc about t
Fix sigmoid_cross_entropy doc about t

functions.sigmoid_cross_entropy 's document has a wrong description about `t`. This PR fixes it.
wkentaro/chainer,aonotas/chainer,chainer/chainer,wkentaro/chainer,okuta/chainer,chainer/chainer,hvy/chainer,niboshi/chainer,ktnyt/chainer,hvy/chainer,niboshi/chainer,keisuke-umezawa/chainer,chainer/chainer,rezoo/chainer,okuta/chainer,jnishi/chainer,tkerola/chainer,wkentaro/chainer,ktnyt/chainer,jnishi/chainer,pfnet/chainer,keisuke-umezawa/chainer,okuta/chainer,okuta/chainer,ktnyt/chainer,keisuke-umezawa/chainer,ktnyt/chainer,jnishi/chainer,niboshi/chainer,hvy/chainer,wkentaro/chainer,hvy/chainer,niboshi/chainer,keisuke-umezawa/chainer,chainer/chainer,anaruse/chainer,ronekko/chainer,jnishi/chainer