RFR: 8264041: Incorrect comments for ParallelCompactData::summarize_dense_prefix

[Albert Mingkun Yang]

Trivial change of fixing comments.

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Commit messages:
 - comment

Changes: https://git.openjdk.java.net/jdk/pull/3152/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3152&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264041
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3152.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3152/head:pull/3152

PR: https://git.openjdk.java.net/jdk/pull/3152