build(deps-dev): bump html-proofer from 3.18.0 to 3.18.2 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.18.0 to 3.18.2.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.18.0...v3.18.2)

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

docs/Gemfile.lock

@@ -28,7 +28,7 @@ GEM
     execjs (2.7.0)
     faraday (1.0.1)
       multipart-post (>= 1.2, < 3)
-    ffi (1.13.1)
+    ffi (1.14.1)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (209)
@@ -84,7 +84,7 @@ GEM
     html-pipeline (2.14.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.18.0)
+    html-proofer (3.18.2)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)