docs: fix custom ruleset filenames in README

[pyrooka]

PR summary

PR Checklist

General checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Dependencies have been updated as needed
• .secrets.baseline has been updated as needed
• npm run update-utilities has been run if any files in packages/utilities/src have been updated

Checklist for adding a new validation rule:

• Added new validation rule definition (packages/ruleset/src/rules/*.js, index.js)
• If necessary, added new validation rule implementation (packages/ruleset/src/functions/*.js, updated index.js)
• Added new rule to default configuration (packages/ruleset/src/ibm-oas.js)
• Added tests for new rule (packages/ruleset/test/*.test.js)
• Added docs for new rule (docs/ibm-cloud-rules.md)
• Added scoring rubric entry for new rule (packages/validator/src/scoring-tool/rubric.js)

[diatrcz]

lgtm!