Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: wenshao

core/src/test/java/com/alibaba/fastjson2/issues_3900/Issue3901.java

@@ -31,7 +31,7 @@ public static class User {
         private String name;
     }
 
-    public class UserReader implements ObjectReader {
+    public static class UserReader implements ObjectReader {
         @Override
         public Object readObject(JSONReader jsonReader, Type fieldType, Object fieldName, long features) {
             jsonReader.readObject();