Bump webrick from 1.8.2 to 1.9.1 in /docs/_spec

Bumps [webrick](https://github.com/ruby/webrick) from 1.8.2 to 1.9.1.
- [Release notes](https://github.com/ruby/webrick/releases)
- [Commits](ruby/webrick@v1.8.2...v1.9.1)

---
updated-dependencies:
- dependency-name: webrick
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

docs/_spec/Gemfile.lock

@@ -41,7 +41,7 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
-    webrick (1.8.2)
+    webrick (1.9.1)
 
 PLATFORMS
   ruby