Fix reference for KAS secret in the docs

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What does this MR do?

Fix typo in the reference to KAS secret. The current reference references the secret key, instead of the secret name.

Following the wrong docs results in a failed deployment of the GitLab chart.

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.