minor update of README and Makefile

[tremes]

This is a minor update to the README link for installing kustomize and a minor update to the Makefile where the undeploy command does not seem to be correct.

I was also wondering if the mention of the kustomize goal in https://github.com/stolostron/multicluster-observability-operator/blob/main/operators/multiclusterobservability/Makefile should be removed, because it doesn't seem to be defined anywhere (and it's referenced in the install, uninstall and deploy goals)

[openshift-ci]

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This pull-request has been approved by: tremes
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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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