Add missing `conversionService` field in doc example

See gh-31330

framework-docs/modules/ROOT/pages/core/validation/convert.adoc

@@ -268,6 +268,8 @@ Java::
 	@Service
 	public class MyService {
 
+		private final ConversionService conversionService;
+
 		public MyService(ConversionService conversionService) {
 			this.conversionService = conversionService;
 		}