Bump webrick from 1.7.0 to 1.8.1 in /docs/_spec

Bumps [webrick](https://github.com/ruby/webrick) from 1.7.0 to 1.8.1.
- [Release notes](https://github.com/ruby/webrick/releases)
- [Commits](ruby/webrick@v1.7.0...v1.8.1)

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

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

Author: dependabot[bot]

Diff for: 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 (1.7.0)
+    webrick (1.8.1)
 
 PLATFORMS
   ruby