Fix retry topic documentation for class-level configuration

[artembilan]

This chapter says:

At this time this functionality doesn’t support class level @KafkaListener annotations.

Which apparently is not true any more according to the info in some next chapter:

Since 3.2, @RetryableTopic support for @KafkaListener on a class would be:

Plus as you see that @KafkaListener has to be a code snippet.