build: bump nokogiri from 1.10.8 to 1.10.9 in /docs

Bumps [nokogiri](https://github.com/sparklemotion/nokogiri) from 1.10.8 to 1.10.9.
- [Release notes](https://github.com/sparklemotion/nokogiri/releases)
- [Changelog](https://github.com/sparklemotion/nokogiri/blob/v1.10.9/CHANGELOG.md)
- [Commits](https://github.com/sparklemotion/nokogiri/compare/v1.10.8...v1.10.9

)

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

docs/Gemfile.lock

@@ -212,7 +212,7 @@ GEM
       jekyll-seo-tag (~> 2.1)
     minitest (5.14.0)
     multipart-post (2.1.1)
-    nokogiri (1.10.8)
+    nokogiri (1.10.9)
       mini_portile2 (~> 2.4.0)
     nokogumbo (2.0.2)
       nokogiri (~> 1.8, >= 1.8.4)