Fix incorrect doc in sparse_softmax_cross_entropy_with_logits

This fix fixes the incorrect doc in sparse_softmax_cross_entropy_with_logits.
In sparse_softmax_cross_entropy_with_logits, `labels` must be `int32` or `int64`,
as is seen in:
https://github.com/tensorflow/tensorflow/blob/46cf73f0214bc6208295e36650f1a8ffde4abdd7/tensorflow/core/ops/nn_ops.cc#L1092-L1099



This fix fixes 21435.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>