build: bump minitest from 5.12.2 to 5.13.0 in /docs

Bumps [minitest](https://github.com/seattlerb/minitest) from 5.12.2 to 5.13.0.
- [Release notes](https://github.com/seattlerb/minitest/releases)
- [Changelog](https://github.com/seattlerb/minitest/blob/master/History.rdoc)
- [Commits](https://github.com/seattlerb/minitest/compare/v5.12.2...v5.13.0

)

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

docs/Gemfile.lock

@@ -212,7 +212,7 @@ GEM
       jekyll (~> 3.5)
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
-    minitest (5.12.2)
+    minitest (5.13.0)
     multipart-post (2.1.1)
     nokogiri (1.10.4)
       mini_portile2 (~> 2.4.0)