📖 docs: fix clusterctl alpha rollout typo

[jdockerty]

What this PR does / why we need it:
Fix a double "l" typo in the command documentation, as kubeadmcontrollplane is not a valid kind to be used by the command.

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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[vincepri]

/approve
/lgtm

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 962b8833831e55677b03eb89f3a5ee378139bf02

[vincepri]

/ok-to-test

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[killianmuldoon]

/cherry-pick release-1.4

[k8s-infra-cherrypick-robot]

@killianmuldoon: once the present PR merges, I will cherry-pick it on top of release-1.4 in a new PR and assign it to you.

In response to this:

/cherry-pick release-1.4

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[k8s-infra-cherrypick-robot]

@killianmuldoon: new pull request created: #8513

In response to this:

/cherry-pick release-1.4

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