Fix a couple of versionadded/versionchanged related markup errors.
diff --git a/Doc/library/mailbox.rst b/Doc/library/mailbox.rst
index 83a590e..8623630 100644
--- a/Doc/library/mailbox.rst
+++ b/Doc/library/mailbox.rst
@@ -89,7 +89,8 @@
       format-specific information is used. Otherwise, reasonable defaults for
       format-specific information are used.
 
-      .. versionchanged:: 3.2 support for binary input
+      .. versionchanged:: 3.2
+         Support for binary input was added.
 
 
    .. method:: remove(key)