Sample App: Use Secret instead of plaintext for slack sink webhook

[Leo6Leo]

docs/code-samples/eventing/bookstore-sample-app/slack-sink/slack-sink-camel-pipe.yaml

Line 20 in 39c5893

webhookUrl: "https://hooks.slack.com/services/YOUR/SLACK/WEBHOOK" # The Webhook URL provided by Slack for posting messages to a specific channel.

Use Secret instead of plaintext for the webhook URL in the slack sink

[asr2003]

/assign

cc: @Leo6Leo

[Leo6Leo]

@asr2003 Hello Ambati, thank you for your willingness to work on this issue! It has been completed already, but I forget to update the issue. Apologize for the inconvenience!