Fix Kotlin example for @Required

Since @Required can only be declared on a method, this commit moves the
@Required declaration from the field to the "set" method.

Closes gh-28590

src/docs/asciidoc/core/core-beans.adoc

@@ -4688,8 +4688,9 @@ example:
 ----
 	class SimpleMovieLister {
 
-		@Required
 		lateinit var movieFinder: MovieFinder
+			@Required
+			set
 
 		// ...
 	}