build(deps-dev): bump socket.io in /showcase/resources/reveal

Bumps [socket.io](https://github.com/socketio/socket.io) from 2.2.0 to 2.4.0.
- [Release notes](https://github.com/socketio/socket.io/releases)
- [Changelog](https://github.com/socketio/socket.io/blob/2.4.0/CHANGELOG.md)
- [Commits](socketio/socket.io@2.2.0...2.4.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]