Fix typos and secret namespace

[mbaldessari]

Without this change the secret never gets propagated to the other clusters. While we're at it, let's fix a couple of typos in the README and also s/secrets/secret/ so kustomize won't error out on this example.

With this change I was able to see the secret propagate to the other clusters. Without it the secret gets created in the default namespace and never gets picked up by the subscription.

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