docs: update the reference link

[SignorMercurio]

Motivation

The reference link in https://pulsar.apache.org/docs/next/reference-configuration is broken because it's using a relative path, but referring to another site that is not managed by Docusaurus.

Modifications

Just a quick fix to replace the relative link to an external link.

Verifying this change

• Make sure that the change passes the CI checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The binary protocol
• The REST endpoints
• The admin CLI options
• Anything that affects deployment

Documentation

• doc-required
  (Your PR needs to update docs and you will update later)

• doc-not-needed
  (Please explain why)

• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)

[SignorMercurio]

@Anonymitaet PTAL