Update 3.15.rst

Author: aisk

Doc/whatsnew/3.15.rst

@@ -692,6 +692,9 @@ Porting to Python 3.15
   :func:`resource.setrlimit` and :func:`resource.prlimit` is now deprecated.
   (Contributed by Serhiy Storchaka in :gh:`137044`.)
 
+* :meth:`~mmap.mmap.resize` has been removed on platforms that don't support the
+  underlying syscall, instead of raising a :exc:`SystemError`.
+
 
 Deprecated C APIs
 -----------------