pw_rpc: CallbackClient test invokes correct method
pw_rpc: CallbackClient test invokes correct method

No-Docs-Update-Reason: Test fix only
Change-Id: If306d429321c162d0380248910d283caf4ec9d18
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/62081
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>
Pigweed-Auto-Submit: Jared Weinstein <96967b8dd0bbdcdfd8a773b034676603aa30f735@google.com>

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