📖 Fix readme wrong url your-own-plugin

[damsien]

Small fix of the README that contains an old url that were working in the book-v3 in the Kubebuilder is also a library section (how to create your own plugins).

The new url is the following: https://book.kubebuilder.io/plugins/extending

[k8s-ci-robot]

Hi @damsien. Thanks for your PR.

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[camilamacedo86]

Thank you 👍

[camilamacedo86]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: camilamacedo86, damsien

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[camilamacedo86]

/lgtm