Update the default room version to 9

Spec PR for [MSC3589](matrix-org#3589)

Signed-off-by: Aaron Raimist <[email protected]>

Author: aaronraimist

content/rooms/_index.md

@@ -51,7 +51,7 @@ stable and unstable periodically for a variety of reasons, including
 discovered security vulnerabilities and age.
 
 Clients should not ask room administrators to upgrade their rooms if the
-room is running a stable version. Servers SHOULD use **room version 6** as
+room is running a stable version. Servers SHOULD use **room version 9** as
 the default room version when creating new rooms.
 
 The available room versions are: