Slack retry header issue

Hi,

In this example under the `slack_bot` lambda, the following code handles slack retry messages:
https://github.com/aws-samples/Multi-Interface-Chatbot-using-Amazon-Q-and-Slack-with-CloudFront-Clickable-References/blob/50d6f6e4aa43c82ca6e1ca1f5d5ae9b94b24f819/lambdas/slack_bot/index.py#L45-L50

However when deploying this example the actual header returned from slack is: `X-Slack-Retry-Num` and not `x-slack-retry-num`.

example from lambda event log:
```json
"headers": {
"Content-Length": "865",
"Content-Type": "application/json",
"User-Agent": "Slackbot 1.0 (+https://api.slack.com/robots)",
"X-Forwarded-For": "3.89.205.0",
"X-Forwarded-Port": "443",
"X-Forwarded-Proto": "https",
"X-Slack-Request-Timestamp": "1739205646",
"X-Slack-Retry-Num": "1",
"X-Slack-Retry-Reason": "http_timeout",
"accept": "*/*",
"accept-encoding": "gzip,deflate"
},
```
the official [slack documentation](https://api.slack.com/apis/events-api#retries) also reference the header as `x-slack-retry-num` so I'm not sure, maybe I got something wrong.