pythongh-117648: Amend NEWS entry (python#117697)

Make the wording more vague; the performance impact varies a lot
depending on platform and input.

Author: erlend-aasland

Misc/NEWS.d/3.13.0a6.rst

@@ -4,7 +4,7 @@
 .. release date: 2024-04-09
 .. section: Core and Builtins
 
-Speedup :func:`os.path.join` by up to 6% on Windows.
+Improve performance of :func:`os.path.join`.
 
 ..