Fix indentation of user-details-service.adoc

Issue gh-7801

docs/manual/src/docs/asciidoc/_includes/servlet/authentication/unpwd/storage/user-details-service.adoc

@@ -1,5 +1,5 @@
 [[servlet-authentication-userdetailsservice]]
-== UserDetailsService
+= UserDetailsService
 
 You can define custom authentication by exposing a custom `UserDetailsService` as a bean.
 For example, the following will customize authentication assuming that `SpringDataUserDetailsService` implements `UserDetailsService`: