DOC: name not defined in section "2.14.1 Basic plotting: plot"
In the [pdf version of document](https://pandas.pydata.org/docs/pandas.pdf), section "User Guide -> Visualization -> 2.14.1 Basic plotting: plot", page 588,  there are `name 'pd' (or 'ts') is not defined` problems.

![image](https://user-images.githubusercontent.com/4179106/102015941-57f69300-3d99-11eb-86c7-28ae8c6a8196.png)
