Update reference to apply

Removed the reference to `apply` as indicated in pola-rs#15945 and replaced with `map_rows` or `map_elements` as appropriate.

py-polars/polars/dataframe/frame.py

@@ -6199,10 +6199,10 @@ def map_rows(
 
         Notes
         -----
-        * The frame-level `apply` cannot track column names (as the UDF is a black-box
+        * The frame-level `map_rows` cannot track column names (as the UDF is a black-box
           that may arbitrarily drop, rearrange, transform, or add new columns); if you
           want to apply a UDF such that column names are preserved, you should use the
-          expression-level `apply` syntax instead.
+          expression-level `map_elements` syntax instead.
 
         * If your function is expensive and you don't want it to be called more than
           once for a given input, consider applying an `@lru_cache` decorator to it.