DOC: fix incorrect pie chart documentation
in pandas-1.2 we used to get a semicircle. However since pandas-1.3 and later the values get rescaled so that they sum up to 1.
This pull request is updating the documentation to reflect this. It looks like matplotlib still draws a partial circle, so this is a change that was made in pandas.

https://pandas.pydata.org/pandas-docs/version/1.2/user_guide/visualization.html#pie-plot
![image](https://user-images.githubusercontent.com/23830955/151903457-57e39718-b549-4513-bc2c-edc7d5600e8d.png)

https://pandas.pydata.org/docs/dev/user_guide/visualization.html#visualization-pie
![image](https://user-images.githubusercontent.com/23830955/151903662-2bb1c12a-2e09-4da8-b3e8-283c48333de5.png)

Could you merge the main branch one more time? > Could you merge the main branch one more time?

I am not sure how to do it. Thanks @partev 