Update clusterctl link in Readme.md

[pierre-emmanuelJ]

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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: pierre-emmanuelJ
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[alvaroaleman]

/ok-to-test

[nikhita]

@pierre-emmanuelJ Welcome to Kubernetes! :)

This PR doesn't really need a release note, can you remove it from the PR body? Thanks!

[scruplelesswizard]

Duplicate of #537
/close

[k8s-ci-robot]

@chaosaffe: Closing this PR.

In response to this:

Duplicate of #537
/close

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