Fix README by adding default entry for new templateResourcePath option in AsciiDoc

[amaechler]

#14909 added a new option templateResourcePath. However, the README was missing a default value for the AsciiDoc table, which caused all the properties after to be rendered wrongly (see the README.adoc and check the properties after templateResourcePath in the rendered Markdown).

This PR adds the None default (to match the existing properties).

PR checklist

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

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  Commit all changed files.
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• File the PR against the correct branch: master (upcoming 7.6.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)