Fixed Typo w/ `data.auth_backends` examples

[aRustyDev]

Description

Relates OR Closes #0000

Checklist

• [ ] Added CHANGELOG entry (only for user-facing changes)
• [ ] Acceptance tests where run against all supported Vault Versions

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

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