fix typo for cert manager installation

[itsmurugappan]

Fixes #2226

Proposed Changes

• Fix typo for cert manager installation

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Welcome @itsmurugappan! It looks like this is your first PR to knative/docs 🎉

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[itsmurugappan]

fixed via #2240. So closing this

[knative-prow-robot]

@itsmurugappan: PR needs rebase.

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