Fix markup for external links.
diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 4cc6d4f..260b3b3 100644
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -679,7 +679,7 @@
:func:`~os.open` function. They can be combined using the bitwise OR operator
``|``. Some of them are not available on all platforms. For descriptions of
their availability and use, consult the :manpage:`open(2)` manual page on Unix
-or `the MSDN <http://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>` on Windows.
+or `the MSDN <http://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windows.
.. data:: O_RDONLY