Use new sso syntax

POSTINSTALL.md

@@ -1,11 +1,12 @@
+<nosso>
 Use the following credentials:
 
 username: `admin`
 
 password: `changeme`
+</nosso>
 
-=== sso ===
-
+<sso>
 Cloudron adminstrators are automatically made WordPress administrators.
 Non-administrators get the `editor` role by default.
-
+</sso>