fix outdated link to spring-security. (#888) (#921)

Update link to Spring Security OAuth 2.0 Resource Server mentioned in Cloud-IAP documentation.

Co-authored-by: Min Zhu <zhumin@google.com>

docs/src/main/asciidoc/security-iap.adoc

@@ -1,12 +1,12 @@
-:spring-security-ref: https://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/
+:spring-security-ref: https://docs.spring.io/spring-security/reference/
 :spring-security-javadoc: https://docs.spring.io/spring-security/site/docs/current/api/org/springframework/security/
 
 [#cloud-iap]
 == Cloud IAP
 
 https://cloud.google.com/iap/[Cloud Identity-Aware Proxy (IAP)] provides a security layer over applications deployed to Google Cloud.
 
-The IAP starter uses {spring-security-ref}#oauth2resourceserver[Spring Security OAuth 2.0 Resource Server] functionality to automatically extract user identity from the proxy-injected `x-goog-iap-jwt-assertion` HTTP header.
+The IAP starter uses {spring-security-ref}servlet/oauth2/resource-server/index.html[Spring Security OAuth 2.0 Resource Server] functionality to automatically extract user identity from the proxy-injected `x-goog-iap-jwt-assertion` HTTP header.
 
 The following claims are validated automatically: