Update html-proofer requirement from 3.17.3 to 3.19.1 (#208)

Updates the requirements on [html-proofer](https://github.com/gjtorikian/html-proofer) to permit the latest version.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](gjtorikian/html-proofer@v3.17.3...v3.19.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Oscar Santiago <[email protected]>

Author: dependabot[bot]

Gemfile

@@ -12,6 +12,5 @@ gem 'asciidoctor-pdf', '1.5.4'
 gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.2.0'
 gem 'thread_safe', '0.3.6'
+gem 'html-proofer', '3.19.1'
 gem 'epubcheck-ruby', '4.2.5.0'
-gem 'html-proofer', '3.17.3'
-