RFR: 8290959: Consistently use "grey" instead of "gray" in GC code

[Kim Barrett]

Please review this trivial spelling change in some comments to make them
consistent with the spelling in identifier names.

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Commit messages:
 - change spelling

Changes: https://git.openjdk.org/jdk/pull/9625/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9625&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290959
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/9625.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9625/head:pull/9625

PR: https://git.openjdk.org/jdk/pull/9625