chore(deps): Bump vuepress-plugin-seo2 in /docs

Bumps [vuepress-plugin-seo2](https://github.com/vuepress-theme-hope/vuepress-theme-hope/tree/HEAD/packages/seo2) from 2.0.0-beta.209 to 2.0.0-beta.251.
- [Release notes](https://github.com/vuepress-theme-hope/vuepress-theme-hope/releases)
- [Changelog](https://github.com/vuepress-theme-hope/vuepress-theme-hope/blob/main/CHANGELOG.md)
- [Commits](https://github.com/vuepress-theme-hope/vuepress-theme-hope/commits/v2.0.0-beta.251/packages/seo2)

---
updated-dependencies:
- dependency-name: vuepress-plugin-seo2
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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