bpo-34124: Fix markup of message_from_binary_file() signature (GH-8297)

(cherry picked from commit c9265c1534b7e62bb9b15460d0420c0c3bb57ff9)

Co-authored-by: Jon Ribbens <jribbens@gmail.com>
diff --git a/Doc/library/email.parser.rst b/Doc/library/email.parser.rst
index dea409d..e0cab6a 100644
--- a/Doc/library/email.parser.rst
+++ b/Doc/library/email.parser.rst
@@ -246,7 +246,7 @@
       Removed the *strict* argument.  Added the *policy* keyword.
 
 
-.. function:: message_from_binary_file(fp, _class=None, *,
+.. function:: message_from_binary_file(fp, _class=None, *, \
                                        policy=policy.compat32)
 
    Return a message object structure tree from an open binary :term:`file