📖 Fix: broken url in external-plugins.md

[monteiro-renato]

Relevant section > https://book.kubebuilder.io/plugins/extending/external-plugins#how-to-write-an-external-plugin

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @monteiro-renato. Thanks for your PR.

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[camilamacedo86]

Could we change to use the right relative paths?

[monteiro-renato]

Hmm, I think the issue here is that we are not pointing to a .md file.
But I can take a look in the mdBook docs to see if it's possible.

[camilamacedo86]

I see that makes sense
we have a way to add code
I would need to give a look, but we are probably not using the core right here
See in other places that we can inject the code on the docs via annotations