Fix note markup (#16805).

Patch by Tshepang Lekhonkhobe, reviewed by Georg Brandl.
diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index c2f5155..be20884 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -644,6 +644,7 @@
    :exc:`UnsupportedOperation`.
 
    .. warning::
+
       :class:`BufferedRWPair` does not attempt to synchronize accesses to
       its underlying raw streams.  You should not pass it the same object
       as reader and writer; use :class:`BufferedRandom` instead.