git: openjdk/leyden: premain: Editorial changes to README, plus better mermaid format from lujaniuk

[John Rose]

Changeset: abdbf473
Author:    John Rose <jrose at openjdk.org>
Date:      2024-06-10 20:18:03 +0000
URL:       https://git.openjdk.org/leyden/commit/abdbf473211c2d985cec21038012280a7b358a46

Editorial changes to README, plus better mermaid format from lujaniuk

! README.md
! test/hotspot/jtreg/premain/lib/GithubMDChart.java