RFR: Update mailman address

[Alexey Ivanov]

Recently, a person sent an email to `mailman-at-openjdk.org` to change the email address as mentioned in the Guide, yet the email was rejected. The rejection email contained another address `mailman-owner-at-openjdk.org` that worked well. Therefore, I suggest updating the mail-man address in the Guide.

I also replaced “discussions _around_” with “discussions _about_”. This is how mailing lists are described on the page that lists all available OpenJDK mailing lists.

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Commit messages:
 - Update mailman address

Changes: https://git.openjdk.org/guide/pull/154/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=154&range=00
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/guide/pull/154.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/154/head:pull/154

PR: https://git.openjdk.org/guide/pull/154