Fix action server tutorial wording (backport #6311)

[mergify]

Summary

• fix the Python action server tutorial to say the ActionServer(...) is added to the node as an action server
• keep the action client wording unchanged in the later ActionClient(...) section

This corrects a confusing typo in the server section where action client was used while describing ActionServer(...).

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This is an automatic backport of pull request #6311 done by [Mergify](https://mergify.com).

[github-actions]

HTML artifacts: https://github.com/ros2/ros2_documentation/actions/runs/23546068765/artifacts/6103366776.

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