📖 Minor fix to configuration.md

[jmnote]

What this PR does / why we need it:

📖 Minor fixed to configuration.md

Instead of duplicating the same command clusterctl upgrade plan, change one side to clusterctl upgrade apply. Ordered according to https://cluster-api.sigs.k8s.io/clusterctl/commands/commands

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

[k8s-ci-robot]

Hi @jmnote. Thanks for your PR.

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[chrischdi]

/ok-to-test

Thanks for contributing!

/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 411686366f3ed1b540a7eab3aa216c0dcc1666e5

[chrischdi]

/area documentation

[sbueringer]

Thx!

/approve

[k8s-ci-robot]

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