docs: fix migration guide public-keys filename

[rsporny]

Description

docs: fix migration guide public-keys filename

Checklist

• Commit sequence broadly makes sense and commits have useful messages.
• New tests are added if needed and existing tests are updated.
• Relevant logging and metrics added
• CI passes. See note on CI.
• Any changes are noted in the changelog.md for affected crate
• Self-reviewed the diff

Note on CI

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You will need to get someone with write privileges. Please contact IOG Partner Chains developers to do this
for you.