RFR: CODETOOLS-7903013: Update README reference to plugin/README

[Jonathan Gibbons]

Please update a minor tweak to the main jtreg README, to update a reference to the plugin README to be a link.

-------------

Commit messages:
 - CODETOOLS-7903013: Update README reference to plugin/README

Changes: https://git.openjdk.java.net/jtreg/pull/25/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=25&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7903013
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/25.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/25/head:pull/25

PR: https://git.openjdk.java.net/jtreg/pull/25