zed: prettify slack notification message

This converts the body of a ZED slack notification from
plain text to code block style to help with readability.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: René Wirnata <[email protected]>
Closes #17610

cmd/zed/zed.d/zed-functions.sh

@@ -441,8 +441,9 @@ zed_notify_slack_webhook()
         "${pathname}")"
 
     # Construct the JSON message for posting.
+    # shellcheck disable=SC2016
     #
-    msg_json="$(printf '{"text": "*%s*\\n%s"}' "${subject}" "${msg_body}" )"
+    msg_json="$(printf '{"text": "*%s*\\n```%s```"}' "${subject}" "${msg_body}" )"
 
     # Send the POST request and check for errors.
     #