docs(duckdb): fix stale memory-limit guidance in 2.1.x accelerator guide

[claudespice]

Summary

The DuckDB accelerator deployment guide for version-2.1.x still told users to set memory via a channel that does not exist — "set a memory_limit pragma via the connection string" (in the Memory sizing note and the OOM on refresh troubleshooting row).

PR #1906 already corrected this fabricated mechanism to the real duckdb_memory_limit acceleration parameter, but that fix landed in vNext and version-2.0.x only and missed the version-2.1.x snapshot, leaving the just-released docs wrong. This propagates the same correction to 2.1.x so all live versions agree.

Verified against code (spiceai/spiceai)

• The DuckDB accelerator exposes duckdb_memory_limit (crates/runtime/src/dataaccelerator/duckdb.rs, ParameterSpec::component("memory_limit")), confirmed present at the v2.1.0 release tag that backs version-2.1.x.
• The accelerator has no connection-string / pragma configuration channel — duckdb_memory_limit is the supported knob, matching what vNext and version-2.0.x already document.

Scope

Version-specific propagation fix — only version-2.1.x was stale (vNext and version-2.0.x were already fixed by #1906), so this touches a single file.

Test plan

• cd website && npm run build passes (prose-only change; no links/tags added)
• Grep confirms zero remaining "connection string" references on the 2.1.x accelerator page
• Wording matches the already-corrected vNext / 2.0.x accelerator pages
• Files updated: 1 (version-2.1.x accelerator deployment guide)

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