fix: correct async-engine docs link slug in README

Fern derives page slugs from the nav title and drops the '&' in
'Architecture & Performance', so the canonical path is
concepts/architecture-performance, not architecture-and-performance.
The latter has no redirect and 404s.

Author: andreatgretel

README.md

@@ -24,7 +24,7 @@ Data Designer helps you create synthetic datasets that go beyond simple LLM prom
 
 ### 📣 Heads-up: async engine is now the default
 
-Data Designer now runs pipelines on a cell-level async engine that overlaps independent columns and adapts concurrency per (provider, model). On most pipelines this is faster with no config changes; on slow self-hosted endpoints, set `inference_parameters.timeout` to your real per-request latency. See [Architecture & Performance → Async Engine](https://docs.nvidia.com/nemo/datadesigner/concepts/architecture-and-performance#async-engine) for the behaviors worth knowing about.
+Data Designer now runs pipelines on a cell-level async engine that overlaps independent columns and adapts concurrency per (provider, model). On most pipelines this is faster with no config changes; on slow self-hosted endpoints, set `inference_parameters.timeout` to your real per-request latency. See [Architecture & Performance → Async Engine](https://docs.nvidia.com/nemo/datadesigner/concepts/architecture-performance#async-engine) for the behaviors worth knowing about.
 
 If you hit anything unexpected, fall back to the legacy sync engine for one transitional release with `DATA_DESIGNER_ASYNC_ENGINE=0`, and please [open an issue](https://github.com/NVIDIA-NeMo/DataDesigner/issues/new) so we can fix the async path.