fix doc for sparse.addmm (#14403)

fix doc for sparse.addmm (#14403)

Summary:
- fixing the doc issue in sparse.addmm

================ before change ==================
![image](https://user-images.githubusercontent.com/38509346/49063994-2f10fe80-f1ce-11e8-9ccc-54241bc45f0b.png)
![image](https://user-images.githubusercontent.com/38509346/49064064-641d5100-f1ce-11e8-865a-7227be7156ef.png)

================ post change ==================
![image](https://user-images.githubusercontent.com/38509346/49064078-76978a80-f1ce-11e8-8f38-f1f8ac9ce63b.png)
![image](https://user-images.githubusercontent.com/38509346/49064085-7bf4d500-f1ce-11e8-8a0d-bf9e5460d21f.png)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14403

Differential Revision: D13216582

Pulled By: weiyangfb

fbshipit-source-id: 52e0a20c6b341c37cfb31f281be3afe2a52ca532