Update html-proofer requirement from 5.2.0 to 5.2.1 (#119)

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)
- [Changelog](https://github.com/gjtorikian/html-proofer/blob/main/CHANGELOG.md)
- [Commits](gjtorikian/html-proofer@v5.2.0...v5.2.1)

---
updated-dependencies:
- dependency-name: html-proofer
  dependency-version: 5.2.1
  dependency-type: direct:production
...

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Author: dependabot[bot]

Gemfile

@@ -14,7 +14,7 @@ gem 'coderay', '1.1.3'
 gem 'pygments.rb', '4.0.0'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '5.3.0.0'
-gem 'html-proofer', '5.2.0'
+gem 'html-proofer', '5.2.1'
 gem 'kindlegen', '3.1.1'
 
 gem 'rouge', '4.7.0'