build(deps): bump vuepress from 1.8.0 to 1.8.1

Bumps [vuepress](https://github.com/vuejs/vuepress/tree/HEAD/packages/vuepress) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/vuejs/vuepress/releases)
- [Changelog](https://github.com/vuejs/vuepress/blob/master/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vuepress/commits/v1.8.1/packages/vuepress)

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

Author: dependabot[bot]