RFR: 8337245: Fix wrong comment of StringConcatHelper

[Shaojin Wen]

8337245: Fix wrong comment of StringConcatHelper

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Commit messages:
 - fix typo

Changes: https://git.openjdk.org/jdk/pull/20344/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20344&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337245
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20344.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20344/head:pull/20344

PR: https://git.openjdk.org/jdk/pull/20344