Correct the signature of mmap.flush in document

Author: aisk

Doc/library/mmap.rst

@@ -212,7 +212,8 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
          Writable :term:`bytes-like object` is now accepted.
 
 
-   .. method:: flush([offset[, size]])
+   .. method:: flush()
+               flush(offset, size, /)
 
       Flushes changes made to the in-memory copy of a file back to disk. Without
       use of this call there is no guarantee that changes are written back before