bpo-42843: Keep Sphinx 1.8 and Sphinx 2 compatibility (GH-24282)

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 048cb2a..aecbec5 100644
--- a/Doc/library/io.rst
+++ b/Doc/library/io.rst
@@ -964,7 +964,7 @@
       .. versionadded:: 3.7
 
 
-.. class:: StringIO(initial_value='', newline='\n')
+.. class:: StringIO(initial_value='', newline='\\n')
 
    A text stream using an in-memory text buffer.  It inherits
    :class:`TextIOBase`.