Difficulty viewing "Lifetime of a PR" diagram in CONTRIBUTING.md if you have a dark background
In CONTRIBUTING.md, the diagram for lifetime of a PR becomes difficult to see when using github dark mode, this is because the text labels and lines within the diagram are black and lack a background color, making them blend in with the dark background

This is how it looks to me:
![image](https://github.com/tensorflow/tensorflow/assets/52792999/c9f2514f-c496-4223-8ef8-fca89347adc3)

Suggested fix:
Add a white background color, for example:
![image](https://github.com/tensorflow/tensorflow/assets/52792999/3eea4ca5-daa0-4570-b0b5-2a2b03a724a3)
