Bump pug from 2.0.4 to 3.0.1 in /03Template-engines/express-example

Bumps [pug](https://github.com/pugjs/pug) from 2.0.4 to 3.0.1.
- [Release notes](https://github.com/pugjs/pug/releases)
- [Commits](https://github.com/pugjs/pug/compare/[email protected]@3.0.1)

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

Author: dependabot[bot]