FE: Fix dummy field in saved JSON message content (#3567)

kafka-ui-react-app/src/components/Topics/Topic/Messages/Message.tsx

@@ -49,7 +49,7 @@ const Message: React.FC<Props> = ({
 }) => {
   const [isOpen, setIsOpen] = React.useState(false);
   const savedMessageJson = {
-    Content: content,
+    Value: content,
     Offset: offset,
     Key: key,
     Partition: partition,