RFR: 8327224: G1: comment in G1BarrierSetC2::post_barrier() refers to nonexistent new_deferred_store_barrier()

[Roberto Castañeda Lozano]

This changeset updates a comment in `G1BarrierSetC2::post_barrier()` to point to the relevant code that must be kept in sync.

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Commit messages:
 - Update comment

Changes: https://git.openjdk.org/jdk/pull/18108/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18108&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327224
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18108.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18108/head:pull/18108

PR: https://git.openjdk.org/jdk/pull/18108