Fix WebFlux authentication reference link

Closes gh-16702

Signed-off-by: Tran Ngoc Nhan <ngocnhan.tran1996@gmail.com>

docs/modules/ROOT/nav.adoc

@@ -128,7 +128,7 @@
 *** xref:servlet/appendix/faq.adoc[FAQ]
 * xref:reactive/index.adoc[Reactive Applications]
 ** xref:reactive/getting-started.adoc[Getting Started]
-** Authentication
+** xref:reactive/authentication/index.adoc[Authentication]
 *** xref:reactive/authentication/x509.adoc[X.509 Authentication]
 *** xref:reactive/authentication/logout.adoc[Logout]
 *** Session Management

docs/modules/ROOT/pages/features/authentication/index.adoc

@@ -8,4 +8,4 @@ Once authentication is performed we know the identity and can perform authorizat
 
 Spring Security provides built-in support for authenticating users.
 This section is dedicated to generic authentication support that applies in both Servlet and WebFlux environments.
-Refer to the sections on authentication for xref:servlet/authentication/index.adoc#servlet-authentication[Servlet] and xref:servlet/authentication/index.adoc[WebFlux] for details on what is supported for each stack.
+Refer to the sections on authentication for xref:servlet/authentication/index.adoc[Servlet] and xref:reactive/authentication/index.adoc[WebFlux] for details on what is supported for each stack.

docs/modules/ROOT/pages/reactive/authentication/index.adoc

@@ -0,0 +1,3 @@
+[[webflux-authentication]]
+= Authentication
+:page-section-summary-toc: 1