Fix AuthenticationPrincipalArgumentResolver xml doc

Fixes gh-3771

docs/manual/src/docs/asciidoc/index.adoc

@@ -6576,10 +6576,13 @@ Spring Security provides `AuthenticationPrincipalArgumentResolver` which can aut
 
 [source,xml]
 ----
-<bean class="org.springframework.security.web.method.annotation.AuthenticationPrincipalArgumentResolver" />
+<mvc:annotation-driven>
+    <mvc:argument-resolvers>
+        <bean class="org.springframework.security.web.method.annotation.AuthenticationPrincipalArgumentResolver" />
+    </mvc:argument-resolvers>
+</mvc:annotation-driven>
 ----
 
-
 Once `AuthenticationPrincipalArgumentResolver` is properly configured, you can be entirely decoupled from Spring Security in your Spring MVC layer.
 
 Consider a situation where a custom `UserDetailsService` that returns an `Object` that implements `UserDetails` and your own `CustomUser` `Object`. The `CustomUser` of the currently authenticated user could be accessed using the following code: