Fix typo in admin role name and heading syntax

The commit fixes two small issues: - Corrects "admind" back to "admin"
in the role list - Changes a level-2 heading to use AsciiDoc syntax

Author: ujibang

docs/default-configuration.adoc

@@ -115,11 +115,11 @@ identityAuthMechanism:
   enabled: false
   username: admin
   roles:
-    - admin
+    - admind
     - user
 ----
 
-## Authenticators
+=== Authenticators
 
 [source,yml]
 ----