RFR: 8353218: Shenandoah: Out of date comment references Brooks pointers

[William Kemper]

Clean backport, trivial change.

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

Commit messages:
 - Backport b4ab964b72c631632511e6f01cdd5a47fb2e31fa

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/185/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=185&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8353218
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/185.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/185/head:pull/185

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/185