Fix inaccurate statement in keda-about.md

[JoeyC-Dev]

Proposed change: Fix misleading statement in limitation section.

Related issues:

• https://github.com/MicrosoftDocs/azure-docs/issues/123886

Supporting point: In the official FAQ, it explicitly says "external" metrics server. If we don't make it clear, this statement will make the misleading like in: https://github.com/Azure/AKS/issues/4436, and cause the user tried to delete the built-in metrics-server. Src: https://keda.sh/docs/2.14/faq/#can-i-run-multiple-metric-servers-serving-external-metrics-in-the-same-cluster

To be clear, there are three types of metrics for Kubernetes. It is important to clarify what kind of metrics should be KEDA unique. Src: https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/#support-for-metrics-apis

[prmerger-automator[bot]]

@JoeyC-Dev : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[Jak-MS]

@tomkerkhove Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

label:"aq-pr-triaged"

@MicrosoftDocs/public-repo-pr-review-team

[tomkerkhove]

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