RFR: Update vestigial comment

[William Kemper]

Update out of date comment pointed out by review of #209 .

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Commit messages:
 - Update vestigial comment

Changes: https://git.openjdk.org/shenandoah/pull/211/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=211&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/211.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/211/head:pull/211

PR: https://git.openjdk.org/shenandoah/pull/211