Update scale-cluster.md

[WayneHoggett-ACG]

Changed from scale up/down to scale in/out.

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[Jak-MS]

@schaffererin 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.

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[schaffererin]

sign-off