fix typo: toDense --> to_dense #25706 (#25832)
Summary:
Only fixes a minor typo in [torch.sparse.FloatTensor docs](https://pytorch.org/docs/stable/sparse.html#torch.sparse.FloatTensor.toDense).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25832
Differential Revision: D17276700
Pulled By: soumith
fbshipit-source-id: cf3d550d5756b000a4e864170ecd4b31826b40f8
