RFR: 8329594: G1: Consistent Titles to Thread Work Items.

[Ivan Walulya]

Trivial; add colon to thread work item titles.  Assists with log parsing.

Testing: Tier1

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/18604/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18604&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329594
  Stats: 12 lines in 2 files changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/18604.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18604/head:pull/18604

PR: https://git.openjdk.org/jdk/pull/18604