[DOCS] Fix realm chains example (#50568)

x-pack/docs/en/security/authentication/realm-chains.asciidoc

@@ -34,32 +34,25 @@ The following snippet configures a realm chain that includes the `file` and
 
 [source,yaml]
 ----------------------------------------
-xpack.security.authc:
-  realms:
-
-    file:
-      type: file
+xpack.security.authc.realms:
+  file.file1:
       order: 0
 
-    native:
-      type: native
+  native.native1:
       order: 1
 
-    ldap1:
-      type: ldap
+  ldap.ldap1:
       order: 2
       enabled: false
       url: 'url_to_ldap1'
       ...
 
-    ldap2:
-      type: ldap
+  ldap.ldap2:
       order: 3
       url: 'url_to_ldap2'
       ...
 
-    ad1:
-      type: active_directory
+  active_directory.ad1:
       order: 4
       url: 'url_to_ad'
 ----------------------------------------