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

diff --git a/Doc/library/asyncio-stream.rst b/Doc/library/asyncio-stream.rst
index 9b456c1..ad3c744 100644
--- a/Doc/library/asyncio-stream.rst
+++ b/Doc/library/asyncio-stream.rst
@@ -185,7 +185,7 @@
       can be read.  Use the :attr:`IncompleteReadError.partial`
       attribute to get the partially read data.
 
-   .. coroutinemethod:: readuntil(separator=b'\n')
+   .. coroutinemethod:: readuntil(separator=b'\\n')
 
       Read data from the stream until *separator* is found.