Issue #26875: Fix mmap example

Patch by Xiang Zhang.

Doc/library/mmap.rst

@@ -127,7 +127,7 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
       import mmap
 
       with mmap.mmap(-1, 13) as mm:
-          mm.write("Hello world!")
+          mm.write(b"Hello world!")
 
    .. versionadded:: 3.2
       Context manager support.