fix small issue on addons readme

[stroebitzer]

What this PR does / why we need it:

link in readme does not work

Which issue(s) this PR fixes:
/kind documentation

Special notes for your reviewer:

Does this PR introduce a user-facing change? Then add your Release Note here:

Documentation:

[kubermatic-bot]

@stroebitzer: Adding the "do-not-merge/docs-needed" label because no documentation block was detected, please follow our documentation process to remove it.

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[kubermatic-bot]

Adding the "do-not-merge/release-note-label-needed" label because no release-note block was detected, please follow our release note process to remove it.

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[kron4eg]

-[addons-docs]: (https://docs.kubermatic.com/kubeone/v1.10/guides/addons/)
-[backups-addon]: (./backups-restic)
-[restic]: (https://restic.net/)
+[addons-docs]: https://docs.kubermatic.com/kubeone/v1.10/guides/addons/
+[backups-addon]: backups-restic/README.md