Correct method signature in code example

Closes gh-1887

src/docs/asciidoc/web/webflux-functional.adoc

@@ -335,7 +335,7 @@ public class WebConfig implements WebFluxConfigurer {
 	}
 
 	@Override
-	default void addCorsMappings(CorsRegistry registry) {
+	public void addCorsMappings(CorsRegistry registry) {
 		// configure CORS...
 	}