DOC: improve pie chart example
#### Location of the documentation

https://pandas.pydata.org/pandas-docs/dev/reference/api/pandas.DataFrame.plot.pie.html

**Note**: You can check the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/).

#### Documentation problem

This example doesn't look great:
![image](https://user-images.githubusercontent.com/33491632/82543164-06f82680-9b4b-11ea-98bf-c7245506410a.png)

The legend covers the labels, and the two pies are so close that "mercury" is hard to read.

#### Suggested fix for documentation

Make a more readable plot