RFR: 7903011: Small tweaks to jtreg plugin documentation

[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

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Commit messages:
 - Update README.md
 - Fix link syntax
 - Update toplevel readme

Changes: https://git.openjdk.java.net/jtreg/pull/24/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=24&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903011
  Stats: 4 lines in 2 files changed: 0 ins; 1 del; 3 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