doc: fix external cluster document

[parth-gr]

fixing the list numbers in external document

closes: https://github.com/rook/rook/issues/13968

Checklist:

• [ ] Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• [ ] Reviewed the developer guide on Submitting a Pull Request
• [ ] Pending release notes updated with breaking and/or notable changes for the next minor release.
• [ ] Documentation has been updated, if necessary.
• [ ] Unit tests have been added, if necessary.
• [ ] Integration tests have been added, if necessary.