docs: fix README SQL reference link

[cloutiertyler]

Description of Changes

The README's SQL reference link returns HTTP 404. Update it to https://spacetimedb.com/docs/reference/sql/, which returns HTTP 200 and contains the SQL Reference content. A repository-wide search found no other occurrences of the broken URL.

Redirect follow-up: configure a permanent 301/308 from /docs/reference/sql-reference to /docs/reference/sql/ in the website's hosting layer. This repository builds static Docusaurus files and uploads them to S3; no permanent HTTP redirect mechanism is configured here. The client-redirect plugin is listed as a dependency but is not enabled. No redirect is included in this patch.

API and ABI breaking changes

None.

Rollback safety impact

n/a

Expected complexity level and risk

1 — a single documentation link change.

Testing

• Verified the old URL returns HTTP 404 and the destination returns HTTP 200 with SQL Reference content, including subscriptions and SQL statements.
• Repository-wide search confirms no occurrences of the broken URL remain.
• corepack pnpm --dir docs build passed, including the configured broken-link checks. Nonfatal warnings covered stale Browserslist data, webpack caching, LLM export of the ask-ai route, and the update check.
• git diff --check passed.