RFR: Add missing JFR mailinglist

[Yang Yi]

Hi, I found that the code owner section of the latest version is missing jfr-related content, this PR will amend this content.

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Commit messages:
 - Add missing JFR mailinglist

Changes: https://git.openjdk.java.net/guide/pull/44/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=44&range=00
  Stats: 3 lines in 1 file changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/guide/pull/44.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/44/head:pull/44

PR: https://git.openjdk.java.net/guide/pull/44