build(deps-dev): bump html-proofer from 3.18.6 to 3.18.8 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.18.6 to 3.18.8.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.18.6...v3.18.8)

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

docs/Gemfile.lock

@@ -31,7 +31,7 @@ GEM
       multipart-post (>= 1.2, < 3)
       ruby2_keywords
     faraday-net_http (1.0.1)
-    ffi (1.14.2)
+    ffi (1.15.0)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (212)
@@ -87,7 +87,7 @@ GEM
     html-pipeline (2.14.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.18.6)
+    html-proofer (3.18.8)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)