Merge pull request #1568 from progit/dependabot/bundler/html-proofer-3.17.3

Update html-proofer requirement from 3.17.2 to 3.17.3

Author: jnavila

Gemfile

@@ -13,4 +13,4 @@ gem 'coderay', '1.1.3'
 gem 'pygments.rb', '1.2.1'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.4.0'
-gem 'html-proofer', '3.17.2'
+gem 'html-proofer', '3.17.3'