build(deps-dev): bump html-proofer from 3.17.1 to 3.17.3 in /docs

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.17.1 to 3.17.3.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.17.1...v3.17.3

)

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

docs/Gemfile.lock

@@ -84,7 +84,7 @@ GEM
     html-pipeline (2.14.0)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.17.1)
+    html-proofer (3.17.3)
       addressable (~> 2.3)
       mercenary (~> 0.3)
       nokogumbo (~> 2.0)
@@ -219,7 +219,7 @@ GEM
     multipart-post (2.1.1)
     nokogiri (1.10.10)
       mini_portile2 (~> 2.4.0)
-    nokogumbo (2.0.3)
+    nokogumbo (2.0.4)
       nokogiri (~> 1.8, >= 1.8.4)
     octokit (4.18.0)
       faraday (>= 0.9)