Update iot-hub.md

[gvuksic]

Links are outdated / not valid anymore.

[ghost]

All CLA requirements met.

[opbld32]

Docs Build status updates of commit 4981dc1:

:warning: Validation status: warnings

File	Status	Preview URL	Details
docs-ref-services/latest/iot-hub.md	:warning:Warning	View (azure-node-latest)	Details

docs-ref-services/latest/iot-hub.md

• Line 5, Column 13: [Warning: ms-service-subservice-invalid - See documentation] Invalid value for 'ms.service': 'iothub'.
• Line 38, Column 68: [Suggestion: docs-link-absolute - See documentation] Absolute link 'https://docs.microsoft.com/samples/browse/?languages=javascript' will be broken in isolated environments. Replace with a relative link.

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