Minor documentation issue on pd.DataFrame.last_valid_index()
Documentation says "first" on both first and last methods

![image](https://user-images.githubusercontent.com/9113853/33384362-f526c1b6-d50c-11e7-97b1-61b8c310301f.png)

![image](https://user-images.githubusercontent.com/9113853/33384382-04d2738a-d50d-11e7-93ac-5c76d3ff1d23.png)

Pandas version = 0.21.0