doc: fix external cluster document (backport #14107)

[mergify[bot]]

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.

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  This is an automatic backport of pull request #14107 done by Mergify.