Update template example in usage doc

[mjwolf]

The example for template.json in the usage doc for the custom template and mapping setting section was incorrect and did not work for both template types. This fixes the example to work with both.

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• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? n/a
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? n/a
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? n/a

Closes #2342