📖 Fix typo in clusterctl generate cluster book docs

[nagygergo]

What this PR does / why we need it:
Minor typo
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 #

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[k8s-ci-robot]

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[sbueringer]

/lgtm
/ok-to-test

Thank you!

[nagygergo]

/assign @neolit123

[neolit123]

/approve

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