Bump thenify from 3.3.0 to 3.3.1 in /demo-site

Bumps [thenify](https://github.com/thenables/thenify) from 3.3.0 to 3.3.1.
- [Release notes](https://github.com/thenables/thenify/releases)
- [Changelog](https://github.com/thenables/thenify/blob/master/History.md)
- [Commits](thenables/thenify@3.3.0...3.3.1)

---
updated-dependencies:
- dependency-name: thenify
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]