Fix errors in documentation

[ericwyles]

What this PR does / why we need it

I corrected a couple of problems in the template-policy.sh in the sigstore-keyless tutorial.

• The jq commands had unnecessary . in the expression causing them not to evaluate
• The sed commands had an unnecessary argument after -i causing the search string to be interpreted as a file name

Acceptance Criteria Met

• [x] Docs changes if needed
• [ ] Testing changes if needed
• [ ] All workflow checks passing (automatically enforced)
• [ ] All review conversations resolved (automatically enforced)
• [ ] DCO Sign-off

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