docs: fix extension name in timescaledb-oss README example

[gbartolini]

Fixes the extension name used in the postgresql.extensions section of the Cluster example in timescaledb-oss/README.md.

The example used timescaledb instead of the correct extension name timescaledb-oss (the metadata name). This now matches the convention used by the other extensions (e.g. pgvector uses name: pgvector under postgresql.extensions and name: vector under the Database resource).

The Database resource example and the \dx verification text are left unchanged, as they correctly refer to the SQL extension name timescaledb (the metadata sql_name).

Closes #224

[jcpunk]

Since the name of the element from ClusterImageCatalog is timescaledb-oss, but the name of the extension for postgresql is timescaledb, I don't think documentation alone will resolve the issue.

CNPG tries to either map timescaledb to an artifact (https://github.com/cloudnative-pg/artifacts/blob/main/image-catalogs-extensions/catalog-minimal-trixie.yaml#L45) and fails to find the extension image or tries to create extension timescaledb-oss. Neither of those will work.

[gbartolini]

The create extension command is driven by what's written in the Database resource though.