Fix postgres/accelerator recipe: correct broken Enterprise feature link

[claudespice]

What the recipe said

postgres/accelerator/README.md line 3 linked the Enterprise requirement at:

https://spiceai.org/docs/enterprise

What the docs do

That path 404s, and no enterprise landing page exists under /docs (checked /docs/enterprise, /docs/features/enterprise, /docs/enterprise/introduction, /docs/getting-started/enterprise — all 404).

The canonical page for this claim is the PostgreSQL Data Accelerator component page, which opens with the Enterprise callout:

Spice.ai Enterprise — The PostgreSQL Data Accelerator is available only in Spice.ai Enterprise.

$ curl -s -o /dev/null -w '%{http_code}' -L https://spiceai.org/docs/enterprise
404
$ curl -s -o /dev/null -w '%{http_code}' -L https://spiceai.org/docs/components/data-accelerators/postgres
200

What changed

Repointed the link to https://spiceai.org/docs/components/data-accelerators/postgres.

The surrounding claim is unchanged and stays accurate against current stable v2.1.1: engine: postgres parses fine, then dataset init calls get_accelerator_engine, which returns None on a community build and surfaces The accelerator engine postgres is not available (crates/runtime/src/init/dataset.rs#L1316-L1320, message at crates/runtime/src/lib.rs#L324-L328).