Fix a typo in Writing-A-Simple-Cpp-Service-And-Client.rst (backport #4934)

[mergify]

The currently code shows error when the result is success.

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This is an automatic backport of pull request #4934 done by Mergify.

[mergify]

Cherry-pick of 3959d21 has failed:

On branch mergify/bp/jazzy/pr-4934
Your branch is up to date with 'origin/jazzy'.

You are currently cherry-picking commit 3959d21.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   source/Tutorials/Beginner-Client-Libraries/Writing-A-Simple-Cpp-Service-And-Client.rst

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/12764683589/artifacts/2427090149.

To view the resulting site:

1. Click on the above link to download the artifacts archive
2. Extract it
3. Open html-artifacts-4937/index.html in your favorite browser