Improve `parallel.mode` chart in User Guide

[Nux]

Current image is unclear. I don't understand what is on X and what is on Y axis. Why is the first example on a diagonal?

https://junit.org/junit5/docs/snapshot/user-guide/images/writing-tests_execution_mode.svg

Seems like this is more accurate, right?

[Marc Philipp]

Thanks for the feedback! Your interpretation is correct. I've updated the chart in cc75ea7.