Fix ResourceSet docs using incorrect "ResourceSetInputsProvider"

[crabique]

PR controlplaneio-fluxcd/distribution#269 was in a wrong place, so this is the correct place.

Using the .kind from the docs verbatim causes this:
ResourceSet/flux-system/test-rset dry-run failed (Invalid): ResourceSet.fluxcd.controlplane.io "test-rset" is invalid: [spec.inputsFrom[0].kind: Unsupported value: "ResourceSetInputsProvider": supported values: "ResourceSetInputProvider", : Invalid value: "null": some validation rules were not checked because the object was invalid; correct the existing errors to complete validation]

[matheuscscp]

LGTM! 🚀