fix(docs): correct GalileoMetric → SplunkAOEvaluator rename in migration guide

[ckazz]

Summary

• SplunkAOMetric does not exist in the codebase; the correct replacement
  for GalileoMetric is SplunkAOEvaluator (src/splunk_ao/evaluator.py)
• Fix splunk-ao-migration-tool/README.md: §3.3 rename table, the
  accompanying diff example, and the §9 migration checklist

Test plan

• Verify SplunkAOEvaluator is exported from splunk_ao (__init__.py line 47)
• Confirm SplunkAOMetric does not exist anywhere in src/

[github-actions]

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[ckazz]

I have read the CLA Document and I hereby sign the CLA

[fercor-cisco]

@ckazz if possible, avoid cross-fork PRs, they make the review process more complicated and introduce trust / security concerns (forks on a public repo may come from external contributors).

Prefer creating a branch directly off https://github.com/splunk/splunk-ao-python if you can.

No changes required for this PR, I am approving it.