Docs: remove localhost links in setup-updating

[HussainAther]

What?

Replaces dev-only http://localhost:3000/... links in docs/setup-updating.md with the correct relative docs link (./setup-creating.html#how-to-subscribe---paid-plans).

Why?

The localhost:3000 URLs are only valid in a specific local dev setup and can break or confuse readers on the published docs site. Relative links work in all environments.

Limitations

• Only updates docs/setup-updating.md.
• Does not change other localhost references elsewhere unless they are similarly dev-only URLs.

Alternatives considered

• Leaving the localhost URLs: risks broken links for most readers.
• Converting to full production URLs: works, but bakes in domain and is less portable than relative links.

Open questions

• None.

[Exairnous]

Thank you for the PR. I have added this to the "review backlog from during sprints" item of the Communication & Documentation Team roadmap Google Doc to ensure this won't be forgotten about.