Rollup merge of #64974 - ecstatic-morse:generic-graphviz-zebra, r=petrochenkov
Rollup merge of #64974 - ecstatic-morse:generic-graphviz-zebra, r=petrochenkov

Fix zebra-striping in generic dataflow visualization

A small formatting improvement to #64828.

Prior to this, the background color of the first row of the table for each basic block changed seemingly at random. You can see this in [basic block #5](https://github.com/rust-lang/rust/pull/64828#issuecomment-536690047) under "New table". Now it is always light.

This also updates the example table to match the current output.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust