Bump html-proofer from 3.8.0 to 3.9.1

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.8.0 to 3.9.1.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](gjtorikian/html-proofer@v3.8.0...v3.9.1)

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

Author: dependabot-support

Gemfile.lock

@@ -84,7 +84,7 @@ GEM
     html-pipeline (2.7.2)
       activesupport (>= 2)
       nokogiri (>= 1.4)
-    html-proofer (3.8.0)
+    html-proofer (3.9.1)
       activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
       colorize (~> 0.8)