[Load Balancing] Fix Origin CA links in zone monitor docs

[gofish]

Summary

Fix documentation for monitors using zone features to link and reference the Origin CA documentation for upstream mTLS instead of the BYOCA docs for downstream/client mTLS.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[cloudflare-docs-bot]

Review

✅ No style-guide issues found in commit f0a54ae.

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