Bump webpack-dev-server from 3.1.5 to 3.9.0 in /_examples/landingpage

Bumps [webpack-dev-server](https://github.com/webpack/webpack-dev-server) from 3.1.5 to 3.9.0.
- [Release notes](https://github.com/webpack/webpack-dev-server/releases)
- [Changelog](https://github.com/webpack/webpack-dev-server/blob/master/CHANGELOG.md)
- [Commits](webpack/webpack-dev-server@v3.1.5...v3.9.0)

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

Author: dependabot[bot]