Update reference documentation for customizer registration.

Replace addBootstrapper(…) with addBootstrapRegistryInitializer(…).

Closes gh-893

Author: mp911de

docs/modules/ROOT/pages/secret-backends.adoc

@@ -560,7 +560,7 @@ You can however enable default registration with
 
 [source,java]
 ----
-public class CustomizationBean implements VaultConfigurer {
+public class MyConfigurer implements VaultConfigurer {
 
     @Override
     public void addSecretBackends(SecretBackendConfigurer configurer) {
@@ -575,7 +575,7 @@ public class CustomizationBean implements VaultConfigurer {
 [source,java]
 ----
 SpringApplication application = new SpringApplication(MyApplication.class);
-application.addBootstrapper(VaultBootstrapper.fromConfigurer(new CustomizationBean()));
+application.addBootstrapRegistryInitializer(VaultBootstrapper.fromConfigurer(new MyConfigurer()));
 ----
 
 [[vault.config.backends.custom]]