bugfix: use the correct key when generating auth example in readme

[chrisimcevoy]

Generates a working example of api key auth in README.md.

cc @antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5

closes #19490

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

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  ./bin/generate-samples.sh ./bin/configs/*.yaml
  ./bin/utils/export_docs_generators.sh
  

  (For Windows users, please run the script in Git BASH)
  Commit all changed files.
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