Update kubernetes_auth_backend_config.md

[Simon-Boyer]

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Description

Adding missing documentation for the backend argument.

Checklist

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

Output from acceptance testing:

N/A. Only changed documentation

...

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[hashicorp-cla-app[bot]]

All committers have signed the CLA.

[Simon-Boyer]

@fairclothjm done! :)

[Simon-Boyer]

Also added what the default is to the description