Add default value for endpoints.cors.max-age

Closes gh-3063

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -602,7 +602,7 @@ content into your application; rather pick only the properties that you need.
 	endpoints.cors.allowed-methods= # comma-separated list of methods to allow. * allows all methods. When not set, defaults to GET.
 	endpoints.cors.allowed-headers= # comma-separated list of headers to allow in a request. * allows all headers.
 	endpoints.cors.exposed-headers= # comma-separated list of headers to include in a response.
-	endpoints.cors.max-age= # how long, in seconds, the response from a pre-flight request can be cached by clients.
+	endpoints.cors.max-age=1800 # how long, in seconds, the response from a pre-flight request can be cached by clients.
 
 	# HEALTH INDICATORS (previously health.*)
 	management.health.db.enabled=true