Update the cluster docs for Knative

What does this MR do?

Fix some wrong versioning and markdown in the new Knative docs.

Related issues

Introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22593/diffs#7fe98eec40e4e9b2adb6c343b19e1e307b8059e8

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8465

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8465