FIX: point the Polars benchmarks link at its final destination (#598)

www.pola.rs redirects to the apex domain; the direct URL avoids the
redirect flagged in the weekly link-check report.

See #597 (closes after the next publish clears the workspace.html
redirect already fixed on main by #590).

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: mmcky

lectures/polars.md

@@ -51,7 +51,7 @@ Polars is designed with performance and memory efficiency in mind, leveraging:
 
 * **Memory**: pandas typically needs 5--10x your dataset size in RAM; Polars needs only 2--4x
 * **Speed**: Polars is 10--100x faster for many common operations
-* **See**: [Polars TPC-H benchmarks](https://www.pola.rs/benchmarks/) for up-to-date performance comparisons
+* **See**: [Polars TPC-H benchmarks](https://pola.rs/benchmarks/) for up-to-date performance comparisons
 ```
 
 Throughout the lecture, we will assume that the following imports have taken place