RFR: 7903011: Small tweaks to jtreg plugin documentation [v2]

[Maurizio Cimadamore]

> I fixed a couple of hiccups in the toplevel and plugin-level README (the issues are described in JBS).
> 
> The updated version of the document can be seen by visiting this branch:
> 
> https://github.com/mcimadamore/jtreg/tree/fix_readme

Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge branch 'master' into fix_readme
 - Update README.md
 - Fix link syntax
 - Update toplevel readme

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Changes: https://git.openjdk.java.net/jtreg/pull/24/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=24&range=01
  Stats: 11 lines in 2 files changed: 3 ins; 7 del; 1 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/24.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/24/head:pull/24

PR: https://git.openjdk.java.net/jtreg/pull/24