<i18n dev> RFR: 8358449: Locale.getISOCountries does not specify the returned set is unmodifiable

[Justin Lu]

Please review this trivial doc correction to `Locale.getISOCountries(Locale.IsoCountryCode type)` which makes it apparent that the returned Set is unmodifiable. Associated CSR filed.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/25622/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25622&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358449
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25622.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25622/head:pull/25622

PR: https://git.openjdk.org/jdk/pull/25622