fix format of bar_of_pie example

[timhoffm]

PR Summary

Rergresion from #11865: formatting issue (missing space after heading).

This resulted in an error when trying to build the docs:

ValueError: Example docstring should have a header for the example title and at least a paragraph explaining what the example is about. Please check the example file:
 bar_of_pie.py

Not sure why this did not appear during CI.